The case notes were studied of 142 mothers admitted to psychiatric hospitals in the south-east Thames region within 12 months of childbirth during the years 1979 and 1980. Only 6% of the sample were categorised as schizophrenics by RDC criteria, whereas affective disorders were found to predominate in 80%. Manic and schizo-affective illnesses almost always began within two weeks of parturition, as did psychotic depressions. A third of the patient sample had suffered relatively minor disorders, and given adequate resources, some might have been better managed in the community. A parallel investigation of diagnostic returns to the Department of Health revealed considerable uncertainty about how to classify puerperal mental illnesses in accordance with ICD-9. There is an urgent need to improve the system for categorising and registering mental illnesses related to childbirth. Until this is achieved, research into aetiology, outcome, and the provision of services will continue to be impeded.