Recent data have revelead that some forms of obsessive-compulsive disorder (OCD) may be characterized by abnormalities of the immune system. Our study aimed at evaluating lymphocytes sub-population in a group of OCD patients, as compared with healthy controls.Fifteen outpatients affected by OCD, according to DSM-IV criteria, and 20 healthy controls were included. Both patients and controls were drug-free and had no physical illness. The OC symptoms severity was assessed with the Yale-Brown Obsessive-Compulsive Scale (Y-BOCS): the mean total score was 20 + 3. Ten ml of venous blood were drawn from fasting subjects for the measurements of lymphocytes sub-sets evaluated with a cytofluorimeter and monoclonal antibodies. The results showed that the patients had a significantly higher number of CD8+ (suppressor) lymphocytes than the healthy controls, while no difference was observed for the other sub-populations.These findings indicate the presence of abnormalities of some components of the immune system in OCD.