Our objective in this study was to determine whether symptoms of obsessive-compulsive disorder (OCD) cluster into groups that can usefully subclassify OCD. Psychiatrists or psychologists interviewed 221 subjects using the Lifetime Anxiety Version of the Schedule for Affective Disorders and Schizophrenia (SADS-LA) for the diagnosis of DSM-IV disorders, and the Yale-Brown Obsessive Compulsive Scale (Y-BOCS) for OCD symptoms. We analyzed 16 symptom categories from the Y-BOCS using exploratory factor analysis to identify latent symptom dimensions. The relationship between these symptom dimensions and clinical characteristics and familiality was investigated. A four-factor model emerged as the best classification of OCD symptoms in the Y-BOCS. These factors were labeled Pure Obsessions, Contamination, Symmetry/Order, and Hoarding. The contamination factor was least likely to be associated with other Axis I disorders. Whereas no significant relationship was found between the factor scores of probands and the presence of OCD in their first-degree relatives, the Symmetry/Order and Hoarding factors did breed true. Hoarding was found to predict poorer treatment response. A four-factor classification of OCD features best describes the symptom patterns of a sample of patients with OCD. There were specific clinical correlates for these factors, and significant intrafamilial sib-sib correlations were found for the Symmetry/Order and Hoarding factors.