This article reports a study of the prevalence of schizophrenia in 3 counties in Ireland based on case-register data and using a standardized diagnostic instrument – the Present State Examination. The estimate of prevalence varies depending on the criteria of schizophrenia employed. A middle-range estimate of 3.9 per 1000 population does not differ substantially from previous estimates of prevalence obtained elsewhere. Over 60% of all schizophrenic prevalence patients and over half of schizophrenic inpatients did not score as cases, suggesting that the continued hospitalization of symptomatically recovered cases has given rise to the mistaken impression that the prevalence of schizophrenia is unduly high in Ireland. The work reported here indicates substantial differences between the results of case ascertainment by hospital admission data compared with those arrived at by standardized interview diagnostic techniques.