To evaluate the efficacy of fluoxetine in the treatment of tics and obsessive-compulsive symptoms in patients with Tourette's syndrome (TS), 14 subjects (8-33 years old) with TS participated in a 20-week, fixed-dose (20 mg daily), double-blind, placebo-controlled crossover trial of fluoxetine monotherapy. Five subjects met criteria for obsessive-compulsive disorder (OCD), 6 additional subjects had obsessive-compulsive features, and 3 subjects had TS without obsessive-compulsive symptoms. There was no improvement in tics after 8 weeks of treatment with fluoxetine (p = 0.58). In contrast, fluoxetine treatment was associated with a significant reduction in obsessive-compulsive symptoms for the group of 6 subjects initially randomized to fluoxetine (p = 0.04). Crossover analysis showed that fluoxetine had no marked effect on tics (n = 10, p = 0.30, but produced a modest decrease in obsessive-compulsive symptoms (n = 8, p = 0.06). Order effects and carry-over effects were not significant. Withdrawal to placebo was associated with a 55% increase in obsessive-compulsive symptoms (p = 0.05), but there was no effect on tics. The most common side effect was transient behavioral activation, which occurred in about half of the subjects and was more common in children. Fluoxetine may be useful for the treatment of obsessive-compulsive symptoms in some patients with TS, but does not appear to be effective for tics.