Until the early 1980s, the only estimate of the prevalence of obsessive compulsive disorder (OCD) in the general population was 0.05%. Data collected from the Epidemiology Catchment Area (ECA) survey have suggested that OCD is 50 to 100 times more common than previously believed and twice as common as schizophrenia or panic disorder in the general population. These results have been corroborated in a second, more carefully designed epidemiologic study. Several reasons account for the previous underestimation of the prevalence of the disorder: (1) reluctance of patients to divulge their symptoms; (2) lack of recognition of the diversity of presenting symptoms in OCD by professionals; (3) misdiagnosis; (4) failure to ask OCD screening questions in the routine mental status examination. Demographic and clinical characteristics of the disorder have been consistent across studies and time, supporting the validity of current nosologic criteria and the disorder's relative homogeneity. OCD appears to be familial, suggesting that genetic factors play a prominent role in the phenotypic expression of illness.