Kaio-ken (界王拳, "Fist of the World King" or "World King Fist") is a fighting technique invented by King Kai. However, Goku is the only person ever able to successfully use it. It is a technique that multiplies the user's ki for a "heart beat", enabling them to inflict serious damage to opponents who are considerably stronger than them. After the introduction of the Kaio-ken, it became one of Goku's signature attacks; however, it became greatly overshadowed by his ability to become a Super Saiyan later on in the animated series and manga. While in the state, the ki aura's color becomes crimson. Later in dragonball Z a new beginning Steve after intensive training with king kai he learned and fully mastered the kaioken.
As a result of the Kaio-ken, the user's base ki level, as well as their strength, speed, and senses, greatly increases for an instant, however, Goku later learns to increase the effect of the Kaio-ken to higher multiplications. When Goku first mastered the Kaio-ken, the furthest he could handle was the Kaio-ken x4, but through training at 100x gravity and then later with his dramatic strength increase after his fight with Captain Ginyu, he was able to withstand astounding multiplications of the Kaio-ken attack, such as Kaio-ken x10. The highest multiplication for the Kaio-ken ever seen was the Kaio-ken x20, which Goku used against Frieza.
The technique however has its substantial flaws, such as if the user multiplies their base ki too much, the uprise of ki could easily obliterate them, or severely damage their body, as it did Goku's; after taking the Kaio-ken x4 to overpower Vegeta's Galick Gun, Goku's entire body was thrashed, and he could barely move without hurting himself. Even one friendly slap on the back from a gracious Yajirobe caused Goku severe pain. King Kai tried to master this technique himself but failed and then taught Goku instead.
The first time Goku used the Kaio-ken was when he used it in the Saiyan Saga to catch up with and cripple Nappa before he could kill Gohan and Krillin with his mouth blast. Later, after Goku realized he couldn't defeat Vegeta even with the Kaio-ken x2, he powered up to Kaio-ken x3, raising his battle strength to near 32,000 and overpowering Vegeta. To prevent Vegeta from destroying the Earth with his Galick Gun, he used a Kaio-ken x3 Kamehameha, a combination of the Kaio-ken x3 and the Kamehameha, in an attempt to save himself and the Earth, initiating an energy wave struggle. When both of their attacks proved unable to overtake the other, Goku used the Kaio-ken x4, surging through Vegeta's Galick Gun and blasting him into the sky. Goku also used this form briefly when he fought Vegeta's Great Ape form, but was whipped aside by Vegeta's tail, surprised at Vegeta for remaining as agile as ever even in the huge size of the Great Ape transformation.
Later, in the Frieza Saga, when Goku fought Captain Ginyu, he demonstrated his Kaio-ken in a prolonged way, increasing his battle strength to a power level around 180,000, thus shocking Ginyu. After being healed in the rejuvenation chamber following his attack from Vegeta when Goku himself had switched bodies with Ginyu, Goku battled with Frieza using a Kaio-ken once again. After realizing Goku was stronger than the others he had battled from Ginyu to Piccolo and the rest and even Vegeta, Frieza used 50% of his maximum power, which gave him an enormous power boost. Goku then had to resort to the Kaio-ken x10, to even have a chance at battling Frieza, but even this wasn't nearly enough. Goku then attempted to use the Kaio-ken x20, which gave him near equal power to the tyrant. Goku eventually used a Kaio-ken x20 and a Kamehameha wave combination, which amazingly, Frieza held off with one hand before stopping it with his own blast, barely hurt. A short time later, Goku eventually transformed into a Super Saiyan, which greatly outclassed the Kaio-ken. However, since--according to battle powers given in the Daizenshuu companion released in Japan--becoming a Super Saiyan increases one's strength fifty times, it is unknown if a Kaio-ken x49 would have equaled the power of a Super Saiyan (if such a Kaio-ken level is even possible to pull off).
Goku's battle against Frieza is the last time the Kaio-ken is used in the manga, as the introduction of Super Saiyan outclassed it. However, in the anime filler fight against Pikkon, Goku used a Kaio-ken and Full-Power Super Saiyan combination (something he calls a "Super Kaio-ken"), which gave him an incredible power increase. Because of this one instance, it appears that Goku can also demonstrated the Kaio-ken's usability in transformed states, which potentially means he can make his transformed forms many times stronger. However, it can be argued that using Kaio-ken in transformed states could seriously injure or even kill him. This is evident in that the already-dead Goku was exhausted after performing the technique for only a few seconds during his fight with Pikkon.
Goku uses the Kaio-ken technique to battle the villains in DBZ movies 2-6.
Goku performing the Kaio-ken in The World's Strongest.Added by LocC 09*In Dragon Ball Z: The World's Strongest, he first uses it to deflate the bio-warrior Misokatsun. He uses it again soon after to free himself after being frozen solid by Ebifurya, and then easily takes out both Ebifurya and Kishime with it. He later uses the Kaio-ken x3 to attack Dr. Wheelo's giant robotic body and succeeds in breaking off Wheelo's right arm. The two then engage in an energy wave struggle similar to the one Goku and Vegeta have in the Saiyan Saga, and, just like in that struggle, Goku overcomes Wheelo's blast by using the Kaio-ken x4.
- In Dragon Ball Z: The Tree of Might, Goku first uses the Kaio-ken to avoid getting hit by the Great Ape-transformed-Gohan, but is eventually knocked down by him. He later uses it to take out Turles' henchmen. After Turles becomes more powerful with the fruit from the Tree of Might, Goku uses the Kaio-ken x10 (Kaio-ken x20 in the FUNimation dub) to attack him, but is still unable to keep up.
- In Dragon Ball Z: Lord Slug, Goku uses the Kaio-ken to rip a hole through the giant Lord Slug, after he gains some of Piccolo's power.
- In Dragon Ball Z: Cooler's Revenge, after Cooler transforms and becomes far more powerful, Goku uses the Kaio-ken to attack him, but has no success. Cooler even flies through Goku's Kaio-ken Kamehameha combo.
- In Dragon Ball Z: The Return of Cooler, Goku again uses the Kaio-ken on Meta Cooler, but it again proves far too feeble, forcing Goku to go Super Saiyan soon after.
Goku has used the Kaio-ken technique in many Dragon Ball video games, usually as a power-up to increase his power.
In Dragon Ball Z: Supersonic Warriors in Krillin's "What-If" story, after dying of heart virus, Goku (with Baba's help) returns to teach the Kaio-Ken & Spirit Bomb techniques to Krillin so he can protect the Earth in Goku's place. Krillin can use these techniques in his level 3 form.
In the upcoming computer game Dragon Ball Online, which is set in 1000 Age, it is said that between the end of the original manga and the start of the game, King Kai taught the Kaio-ken skill to the inhabitants of Earth. This included: Humans, the planet's original inhabitants, who since the manga's end, have began practicing different forms of martial arts; and the Namekians, who fled to Earth after their planet was destroyed by Miira. The Majin race, who were not taught the technique, managed to learn it through mimicry, similar to how Majin Buu learned Goku's Kamehameha by witnessing it.