The aim of this study was to explore the possible relationship between plasma clomipramine and its major metabolite (N-desmethylclomipramine) levels and related parameters, and clinical features in patients with obsessive-compulsive disorder (OCD). Twenty-six outpatients (13 men, 13 women), suffering from OCD were consecutively enrolled in this study. The severity of OCD was assessed by the Yale-Brown Obsessive Compulsive Scale (Y-BOCS). The measurements were taken after 4 weeks and 6 months from the beginning of the treatment. The drug levels were measured by a high-performance liquid chromatography method developed by us. The correlations between biological and clinical parameters were analyzed by means of Spearman's correlation coefficient. The Mann-Whitney test was used for comparing biological and clinical variables between men and women. The results showed that clomipramine levels were related to the doses at the two assessment times. A significant and positive correlation was detected at the beginning between the N-desmethylclomipramine ratio and the Y-BOCS total score; however, this was true only for men, where the similar correlations were measured also with the Y-BOCS subscale. After 6 months of clomipramine, men showed a significant improvement of the compulsions. These findings would highlight the potential impact of assessing clomipramine plasma levels and their relationships with specific symptoms, as well as the influence of the sex on the drug response.