A case note study of postpartum psychiatric illness as defined by Research Diagnostic Criteria (RDC) was conducted in an epidemiologically defined large area of Japan. The admission rate was 0.34/1000 live births. The most frequent diagnostic category was affective disorder (53%). "Atypical" symptoms were observed in 31% of all cases and were more frequent (67%) in patients with schizoaffective disorder. Fifty six percent of mothers developed a psychiatric illness within two weeks of delivery. The patients with "atypical" symptoms were admitted much more quickly and their length of stay in hospital was shorter.