Response to SSRIs suggests the implication of the serotonergic system in obsessive-compulsive disorder (OCD). However, biological studies on serotonergic function in OCD have yielded contradictory results. Platelet monoamine oxidase (MAO) activity has been proposed as an index of cerebral serotonin activity. The aim of this study was to examine platelet MAO activity in 29 OCD patients and 29 healthy controls matched by age, sex and tobacco use. We also explored the relationship between platelet MAO activity and aggressive obsessions in OCD patients. There were no differences in platelet MAO activity between OCD patients and healthy controls. We found a significant correlation between platelet MAO activity and Y-BOCS scores in the group of patients with Y-BOCS scores >15. OCD patients with aggressive obsessions had significantly lower levels of platelet MAO activity than patients without aggressive obsessions. Our results suggest that platelet MAO activity may be a marker of OCD severity, and that low platelet MAO activity may be associated with aggressive obsessions in OCD patients.