Objective. The clinical characteristics and response to pharmacotherapy of adult patients with early-onset and late-onset obsessive-compulsive disorder (OCD) were compared in this study. Methods. A total of 50 outpatients with OCD diagnosed according to DSM-IV criteria (early-onset: 20; late-onset: 30) were included in the study. After initial clinical evaluation with The Structured Clinical Interview for DSM-IV/Clinical Version (SCID-I/CV), The Structured Clinical Interview for DSM-III-R Personality Disorders (SCID-II) and Yale-Brown Obsessive-Compulsive Scale (Y-BOCS), all patients were treated with fluvoxamine, sertraline or paroxetine for 12 weeks. Treatment response was defined as a >= 35% reduction in the Y-BOCS-total scores from baseline in a 12-week follow-up period. Results. Forty-three patients (early-onset: 16; late-onset: 27) completed the study. The early-onset group had higher frequencies of symmetry/exactness obsessions and ordering/arranging compulsions, and the late-onset group had higher mean age at assessment. Nine (56.3%) patients with early-onset and 18 (66.7%) with late-onset responded to pharmacotherapy. The difference between response rates was not statistically significant. Conclusions. Our study suggests that although there are some phenomenological differences between patients with early-onset OCD and late-onset OCD, these patients have similar responses to pharmacotherapy.