For the investigation of the early course of schizophrenia starting from onset, the standardised Interview for the Retrospective Assessment of the Onset of Schizophrenia was developed and validated. In a representative sample of 267 first-admitted German schizophrenics of a broad diagnosis from a population of 1.5 million, the age at which different diagnostic and onset definitions were satisfied, the symptoms at the time of the interview, and the accumulation of positive and negative symptoms until first admission were assessed. Comparison between the two sexes and three age groups yielded hardly any differences in the accumulation of symptoms and their course until first admission, except for a slightly shorter period of negative symptoms in young males and a slightly longer one in older women – which contradicts prevailing opinion. At the time of the interview, no significant sex differences were found with respect to the core symptoms of schizophrenia (negative and first-rank symptoms), but clear and substantial differences emerged in disease behaviour. The significantly higher age at first onset in women is explained, on the basis of animal experiments and a clinical study, by the neuromodulatory effect of oestrogen on D2 receptors and by a higher vulnerability threshold in women.