In the article the problems of prevalence of obsessive-compulsive disorder (OCD), comorbidity with other mental disorders and the most commonly used diagnostic instruments were presented. Epidemiological data indicate OCD as the fourth most common mental disorder, which appears as frequent as asthma or diabetes. In an international study from 1994 the year's prevalence of OCD was from 1.1% to 1.8%. In each country obsessions and compulsions appear with different intensity, what can testify for certain cultural conditionings of disorder. Men suffer from OCD equally often as women, however both first symptoms and full symptom disorder appear several years earlier in men than in women. The prevalence of OCD among children is from 2% to 4%. At least half of the patients suffer from OCD also in maturity. OCD should be differentiated from generalized anxiety disorder, panic disorder, phobias, anancastic personality and hypochondria.