Obsessive-compulsive disorder (OCD) is a common disorder in adolescents, usually treated in the outpatient setting. Our aim in this study was to evaluate the clinical characteristics of adolescents with severe OCD that required hospitalization. A total of 342 patients consecutively admitted to a psychiatric adolescent inpatient unit and 87 healthy volunteers were assessed by a semistructured interview for clinical diagnosis, suicide risk factors, aggression, ego defense mechanisms, and intelligence. Patients with OCD (n=40) were compared to other four diagnostic patient groups with psychotic, affective, conduct, and eating disorders, as well as to normal controls. Adolescent inpatients with OCD experienced less separation anxiety than all the other psychiatric groups (P < .01) and were less impulsive than controls (P < .001). They differed in aggressive/impulsive traits and hospital-related behaviors from other diagnostic groups. Adolescent inpatients with OCD consist of a unique subgroup in the inpatient unit in terms of their clinical characteristics and risk factors for suicide. These characteristics should be taken into account when developing a treatment plan for these difficult-to-treat inpatients.