The serum calcium of 53 recently delivered mothers hospitalized for severe puerperal psychiatric illness, which represented the whole intake from a defined catchement area, was compared with that of 35 female psychiatric patients and that of 49 normal postnatal women. The mean corrected and ionized serum calcium values of the puerperal psychiatric patients with no personal or family history of psychiatric disorder were markedly above the normal range. They were also significantly higher than those of the puerperal psychiatric patients with a personal or family history of pscychiatric illness and those of the two control groups. There was a modest positive correlation between the degree of hypercalcemia and the severity of the psychiatric illness. The follow-up of 16 puerperal psychiatric patients indicated that the fall in ionized serum calcium levels correlated positively and significantly with the improvement in rated symptomatology. Patients with severe puerperal psychiatric disorder can be divided etiologically into two groups. The larger proportion is psychiatrically vulnerable, but in the remainder (about a third of the total number), the psychiatric illness appears to be related to a disorder of calcium homeostasis in the puerperium.