In Season 4, Booth develops a brain tumor which makes him hallucinate. In "The Critic in the Cabernet", Booth sees Stewie Griffin (who is originally a character on the animated television series Family Guy) telling him he is not man enough to donate sperm for Brennan and makes Booth feel queezy. He also sees him in the interrogation room and talks back to him in the presence of Brennan, who realizes that Booth has a problem. She therefore persuades him to go to the hospital to get checked out and they find out that he has a brain tumor that made him hallucinate. This was the only episode we see Stewie in and the last episode in which Booth hallucinates before surgery.