In a context of a WHO collaborative study, 12 research centres in 10 countries monitored geographically defined populations over 2 years to identify individuals making a first-in-lifetime contact with any type of 'helping agency' because of symptoms of psychotic illness. A total of 1379 persons who met specified inclusion criteria for schizophrenia and other related non-affective disorders were examined extensively, using standardized instruments, on entry into the study and on two consecutive follow-ups at annual intervals. Patients in different cultures, meeting the ICD and CATEGO criteria for schizophrenia, were remarkably similar in their symptom profiles and 49% of them presented the central schizophrenic conditions as defined by CATEGO class S+. However, the 2-year pattern of course was considerably more favourable in patients in developing countries compared with patients in developed countries, and the difference could not be fully explained by the higher frequency of acute onsets among the former. Age- and sex-specific incidence rates and estimates of disease expectancy were determined for a 'broad' diagnostic group of schizophrenic illness and for CATEGO S+ cases. While the former showed significant differences among the centres, the differences in the rates for S+ cases were non-significant or marginal. The results provide strong support for the notion that schizophrenic illnesses occur with comparable frequency in different populations and support earlier findings that the prognosis is better in less industrialized societies.