It has become apparent in recent years that obsessive compulsive disorder (OCD) is much more common than was previously recognised. According to the results of the Diagnostic Interview Schedule used in the Epidemiological Catchment Area Study, OCD affects 1.3% of adults in the U.S. Community psychiatry surveys with the Present State Examination in Camberwell, South London, and in the Dutch Health Area of Nijmegen yielded an OCD prevalence of 10.8% and 7.3% respectively. The author discusses the epidemiological data on OCD and explores possible reasons for some divergent results.