The present study examines the effect of concomitant major depressive or bipolar disorder on clinical symptoms of patients with obsessive-compulsive disorder (OCD). Forty-nine patients classified as OCD without a mood disorder, 26 classified as OCD with bipolar disorder (OCD-BD) and 42 classified as OCD with major depressive disorder (OCD-MDD) according to DSM-IV diagnostic criteria were included in the study. The groups were compared with respect to demographic variables and scores obtained on various scales. The OCD-BD group had more symmetry/exactness obsessions and ordering/arranging compulsions, and a more episodic course of illness and had better insight compared to the other two groups. Levels of anxiety, depression, disability and obsessive-compulsive symptom severity were significantly higher in the OCDMDD group. The rate of social phobia was higher in OCD-BD patients, whereas the rates of generalized anxiety disorder and simple phobias were higher in OCDMDD group. These findings suggest that comorbidity of major depressive disorder may increase the severity of OCD symptoms. On the other hand, bipolar disorder comorbidity may constitute a subgroup which is characterized by a higher rate of episodic course and better insight.