Stuttering is a speech disorder that affects one-percent of all adults and much has been learned recently of its neurologic correlates. Stuttering has been associated with excessive cerebral activity of the neurotransmitter, dopamine. Pharmacologic research has suggested that older generation dopamine antagonist (i.e. "typical antipsychotic") medications improve stuttering symptoms, but are associated with poorly tolerated adverse effects. The purpose of this study was to compare the efficacy and tolerability of olanzapine, a novel dopamine antagonist (or "atypical antipsychotic"), versus placebo in the treatment of adult developmental stuttering. Twenty-four adults who stutter participated in a twelve-week, randomized, double-blind, placebo-controlled trial conducted at two separate sites. Subjects received either olanzapine (2.5 mg titrated to 5 mg) or matching placebo. Subjects were rated on an objective measure of stuttering severity (SSI-3), a clinician based global impression (CGI), and a subject-rated self-assessment of stuttering (SSS). Subjects were also monitored for potential side-effects. Twenty-three of the twenty-four subjects enrolled in the trial successfully completed the full course of the study. Olanzapine was statistically superior to placebo on the three ratings of stuttering severity, the SSI-3, the CGI and SSS (p < .05). Olanzapine is a promising medication for the treatment of stuttering and further research is warranted.