The study evaluated the prevalence of DSM-IV-TR-defined psychiatric disorders in adolescents with mental retardation, with a focus on obsessive-compulsive disorder (OCD), for which data at present are sparse. Eighty-seven adolescents with mild to moderate mental retardation attending the Israeli special-education system were screened for psychiatric disorders in general and obsessive-compulsive symptoms in particular. Sixty-one percent had at least one psychiatric disorder. Of the 13 participants receiving antipsychotic medication, none had an underlying psychotic disorder and most had anxiety or depressive disorders. OCD was detected in 11% of participants and was characterized by high rates of psychiatric comorbidities. The severity of autistic symptoms predicted 39% of the variance in the severity of OCD symptoms. Adolescents with mild to moderate mental retardation have high rates of psychiatric morbidities that are often inappropriately treated. OCD is prevalent in this population and is strongly associated with autistic symptoms. Further studies are required in adolescents with mental retardation to better delineate psychiatric morbidities and their appropriate treatment in this at-risk population.