Serotonergic mechanisms have been implicated in panic disorder, and several preliminary studies suggest that fluvoxamine, a selective serotonin reuptake inhibitor (SSRI), is helpful in its treatment. This 8-week double-blind parallel-group study compared fluvoxamine with a placebo in 188 patients with DSM-III-R defined panic disorder with or without agoraphobia. Efficacy assessments included a Daily Panic Attack Inventory, the Sheehan Disability Scale, the Clinical Anxiety Scale and the Clinical Global Impression Scale. When compared with the placebo, fluvoxamine produced highly significant improvements in most measures of the frequency and severity of panic disorder and in the more global aspects of disability and distress. Significant improvement was evident as early as week 1 for some panic variables. Fluvoxamine is a potent anti-panic agent with a relatively rapid onset of action.