Data from two sources-the Edinburgh Psychiatric Case Register and the psychiatric inpatient records of the Scottish Health Service-were used to compare large populations of first-admission schizophrenics born in winter (January to March) and in summer (June to October). Parallel comparisons were carried out for affective psychoses. Comparison of the months of birth of the Scottish patients with those of the general population indicated that there was a 9% excess of schizophrenic births and a 3% excess of affective births in the first 3 months of the year. In the Edinburgh material, winter-born schizophrenics were more likely than the summer-born to receive a diagnosis of paranoid or schizoaffective schizophrenia and less likely to receive diagnoses other than schizophrenia on readmission, but neither of these differences emerged in the much larger Scottish material. There were no differences between winter-and summer-born schizophrenics in age of onset, sex ratio, or prognosis in either data set, nor were any significant differences found between winter- and summer-born affectives. We have therefore failed to demonstrate any convincing differences between winter-and summer-born schizophrenics.