The quarterly distribution of births of patients born in England and Wales 1921–60 and first admitted in 1970–75 was examined by decade of birth and by age at year of admission. For patients with schizophrenia and affective psychosis, the distribution varied: in the early decade (1921–30), and for older patients (45–54 years) the proportion of births in the fourth quarter of the year was high, compared with expectation from live births in the general population; but it became lower in succeeding decades and for younger age groups. No comparable change occurred for births of patients with neurosis or personality disorder.