In general, results of epidemiological studies on schizophrenia performed during the last century have the remarkable feature that no significant differences in expectancies and incidences have been found, neither between different populations nor over time. Recently, however, some studies have tended to show a noteworthy decrease in the incidence, as measured by statistics of first admissions. Possible sources of error are discussed, partly on the basis of recent Danish investigations. Although it is uncertain whether a true change in the incidence of schizophrenia has occurred, one feature is becoming evident: under modern treatment and care of-schizophrenics the natural history of the disease is becoming increasingly different between the two sexes. This fact, in combination with one of the hardest facts in schizophrenia research, namely the higher age of onset in females, may be useful starting points for hypotheses on aetiology, or at least necessary components of such hypotheses.