The Kaio-ken ("Fist of the Worlds") or the Kaio-Ken Attack was a technique invented by King Kai, in which Goku was the only person ever able to successfully use the technique. In the series, the Kaio-ken is a technique that multiplies the user's ki for a "heart beat", enabling them to inflict serious damage to foes 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.
As a result of the Kaio-ken, the user's base ki level doubles for an instant, however, Goku later learns to increase the effect of the Kaio-ken to higher multiplications. The topmost multiplication for the Kaio-ken ever seen was the Kaio-ken x20, which Goku used against the (at the time) nearly omnipotent Frieza. The technique also has its substantial flaws, such as if the user multiplies their base ki too much, or if their bodies are too severely damaged, the uprise of ki could easily obliterate them. 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 and the Kaio-ken X20.
The first time Goku used the Kaio-ken was when he used it in the Saiyan Saga to catch up with Nappa before Gohan and Krillin are taken out by his mouth blast. He multiplied his power and speed and broke Nappa's back, disabling him. 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 24,000, overpowering the Saiyan prince, and then, 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 a ki beam struggle. When both their attacks proved unable to overtake the other, Goku used the four fold Kaio-ken, 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.
Later, in the Frieza Saga, when Goku fought Captain Ginyu, he demonstrated his Kaio-ken X2, raising his battle strength to above 180,000, shocking even the captain. 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 X10. After realizing Goku was stronger then the others he had battled, Frieza used 50% of his maximum power, which gave him a enormous power up. 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. A short time later, Goku eventually transformed into a Super Saiyan, which greatly outclassed the Kaio-ken. However, since--according 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 x50 would have equaled the power of a Super Saiyan (if such a Kaio-ken level is even possible). While on Yardrat, Ryun used the Kaio-ken to fight off Kuriza's soldiers. He never really used his full power against them, but it outclassed them easily anyway. During the fight against Evil Super Saiyan 4 SVX, Goku Jr used the Kaio-ken as a bluff saying that he could transform into a Super Saiyan 5. Goku Jr had to escalate to 5x Kaio-ken to stand a chance against the brutal Saiyan.
Goku also uses this move in Movies two through six. The enemies are listed below:
- He uses this against Dr. Wheelo and his cyborg warriors in The World's Strongest. He defeats both opponents he uses this move, although he kills Dr. Wheelo with a Spirit Bomb.
- He battles Turles with the Kaio-ken in The Tree of Might, but after eating from the tree's fruits, Turles easily turns the tables, even when Goku uses the Kaio-ken x 10.
- In Lord Slug, Goku rips a hole through the giant Namekian, when he gains some of Piccolo's power, and uses the Kaio-ken on several occasions.
- In Cooler's Revenge, Goku fights Cooler, Frieza's older brother. Although at first having the upper hand, Goku loses his advantage against Cooler's final form, being grossly over powered. Cooler even flies through Goku's Kaio-ken X20 Kamehameha combo, the one that caused his brother quite some pain.
- Goku's battle against Frieza is the last time the Kaio-ken is used in the series, as the introduction of Super Saiyan outclassed it. However, in the filler fight against Pikkon, Goku uses the Kaio-ken and Full-Power Super Saiyan combination (something he calls a Super Kaio-ken), which gave him an incredible power increase and demonstrated the Kaio-ken's usability in transformed states, which potentially means he can make his transformed forms (including Super Saiyan 4) 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. This "Super Kaioken" was only used once and it punched Pikkon straight through the moon orbiting the tournament stage. This technique only appeared in a filler segment of the anime, and never in the original manga, so according to many fans, it is not canon.