Because they found in an earlier study that inositol, an important intracellular second-messenger precursor, was effective against depression in open and double-blind trials, the authors studied its effectiveness against panic disorder. Twenty-one patients with panic disorder with or without agoraphobia completed a double-blind, placebo-controlled, 4-week, random-assignment crossover treatment trial of 12 g/day of inositol. The frequency and severity of panic attacks and the severity of agoraphobia declined significantly more after inositol than after placebo administration. Side effects were minimal. The authors conclude that inositol's efficacy, the absence of significant side effects, and the fact that inositol is a natural component of the human diet make it a potentially attractive therapeutic for panic disorder.