OBJECTIVE: Earlier studies reported that inositol, a simple polyol second messenger precursor, was effective in controlled trials for patients with depression and panic. In this study its effectiveness in obsessive-compulsive disorder was investigated. METHOD: Thirteen patients with obsessive-compulsive disorder completed a double-blind, controlled crossover trial of 18 g/day of inositol or placebo for 6 weeks each. RESULTS: The subjects had significantly lower scores on the Yale-Brown Obsessive Compulsive Scale when taking inositol than when taking placebo. CONCLUSIONS: The authors conclude that inositol is effective in depression, panic, and obsessive-compulsive disorder, a spectrum of disorders responsive to selective serotonin reuptake inhibitors.