OBJECTIVE: The authors examined the distribution of catechol O-methyltransferase (COMT) genotypes in patients with panic disorder as well as the relationship between a COMT polymorphism and the clinical characteristics of these patients. METHOD: Fifty-one patients with panic disorder and 45 healthy comparison subjects were tested for a genetic polymorphism of COMT. Clinical variables were assessed for the patients with panic disorder. RESULTS: The frequency of the L/L genotype was significantly higher in the patients with panic disorder than in the healthy subjects (19.6% versus 2.2%). Panic disorder was significantly associated with the L allele and L/L genotype. Patients with panic disorder who had the L/L genotype showed poorer treatment response than those with other genotypes. CONCLUSIONS: These results suggest that the L/L genotype of the COMT gene may be related to the development and treatment outcome of panic disorder in some patients.