The objective of this study was to investigate axis I comorbidity in obsessive-compulsive disorder (OCD). A total of 147 patients diagnosed as OCD for DSM-III-R criteria were included in the study. At least one comorbid axis I disorder was present in 68.7% of the patients. Major depression was the most common comorbid disorder (39.5%). Uncomplicated (n = 46) and comorbid (n = 101) OCD groups were compared with respect to the demographic variables and the scores obtained from the Hamilton Rating Scale for Anxiety (HRSA), the Hamilton Rating Scale for Depression (HRSD) and the Yale-Brown Obsessive Compulsive Scale (Y-BOCS). No significant difference in terms of age, sex, marital status, age at onset, or duration of illness was found between the groups. The scores on the HRSA, HRSD, and Y-BOCS were significantly higher in the comorbidity group. Our results demonstrate that at least one psychiatric disorder is present in approximately two thirds of OCD patients, the majority being depressive disorders and anxiety disorders. The fact that comorbid conditions raise not only anxiety and depression levels, but also the severity of obsessions and compulsions, is noteworthy. Copyright 2002, Elsevier Science (USA). All rights reserved.