A randomized, double-blind, placebo-controlled study was conducted to assess the efficacy of risperidone in the treatment of developmental stuttering in 16 adults. Eight subjects received placebo and eight received risperidone at 0.5 mg once daily at night, increased to a maximum of 2 mg/day. After 6 weeks of treatment, decreases in all measures of stuttering severity were greater in the risperidone group than in the placebo group; the between-treatment difference was significant (p < 0.05) on the most important measure, the percentage of syllables stuttered. In the risperidone group, reductions from baseline in scores for the percentage of syllables stuttered, time stuttering as a percentage of total time speaking, and overall stuttering severity were significant (p < 0.01); changes in scores on the fourth measure of stuttering, duration, were not significant. No significant decreases occurred in the placebo group. Among the eight patients in the risperidone group, five responded best to 0.5 mg/day, with stuttering recurring at higher doses. The remaining three patients responded better with increasing doses of risperidone. Risperidone was generally well tolerated. The results of this small study indicate that risperidone may be effective in the treatment of developmental stuttering. This finding needs to be confirmed in a larger trial.