An open-label, 8-week, follow-up study was designed to compare subjects with child versus adult onset of obsessive compulsive disorder (OCD) on their clinical characteristics and response to a standardized treatment with clomipramine. The study included 15 children and 13 adults with Diagnostic and Statistical Manual of Mental Disorders, 4th edition (DSM-IV) diagnosis of OCD. Children received 50-225 mg/day (3-5 mg/kg day) and adults 150-250 mg/ day of clomipramine. The children's group had a higher number of males and increased comorbidity than the adults' group. A similar dose/kg day of clomipramine was effective in reducing OCD severity and improving the psychosocial functioning in both groups; however, total improvement according to the National Institute of Mental Health-Global Obsessive Compulsive Rating Scale (NIMH-GOCRS) was greater in adults. Adults reported a higher frequency of side effects compared to children, with dry mouth and constipation more common at the end of the study. These data suggest that children with OCD seeking treatment may have differences from adults and support the importance of studying age-of-onset subtypes of OCD.