OBJECTIVE: Obsessive-compulsive disorder (OCD) is a clinically heterogeneous disorder with a bimodal pattern in age onset and treatment outcomes. This study attempted to ascertain the importance of the age factor for a better phenotypic precision. Therefore, the authors compared adult OCD patients with an early symptom onset to adult OCD patients with a later symptom onset. METHODS: One hundred sixty five patients with OCD were evaluated with semistructured interviews;79 with symptom onset before the age of 17 (early onset group) and 86 with symptom onset after the age of 17 (late onset group). The two groups were analyzed in terms of Y-BOCS (Yale-Brown Obsessive Compulsive Scale) scores and demographic data including clinical variables. RESULTS: Early onset group has more comorbidity of tic disorder and lesser of depression and anxiety disorder than late onset group. Early onset group showed more family history of tic disorder than late onset group. The treatment response to SSRI is relatively declined after 18 months of initiation in early onset group. CONCLUSION: The results indicate that age at onset may be an important factor in subtyping OCD. Early onset group may have more biological and familial tendency that might be differentiate the two groups.