In March 1998, Yamaguchi-san signed with the World Wrestling Federation (WWF). He recruited Sho Funaki, Mens Teioh, and Dick Togo to form the stable Kaientai, who feuded with Taka Michinoku. They defeated Michinoku and Bradshaw at Over the Edge: In Your House. Following this, the stable began a storyline with Val Venis, who had been sleeping with Yamaguchi-san's wife, Kiyoko, with an enraged Yamaguchi-san being shown a video of the incident. The following week, during a Val Venis match, Yamaguchi-san cut a promo in which he chopped a salami in two with a sword as a threat to Venis that he would "choppy choppy his pee pee".
Taka Michinoku turned on Venis during a tag team match against Togo and Funaki, effectively turning heel and joining Kaientai. Mrs. Yamaguchi was also later revealed to be Michinoku's sister. Venis was dragged backstage to seemingly have his penis amputated, which led to the infamous "castration" scene on the August 3 episode of Raw is War. Venis later explained that Yamaguchi's sword narrowly missed his penis, owing to a "little shrinkage" and some timely assistance from "friend" John Wayne Bobbitt.
Yamaguchi, Togo and Teioh were later granted their release in late 1998, while Taka and Funaki stayed and became a regular tag team.
On March 9, 2019, Yamaguchi-san passed away.