We investigated the value of a screening program for postpartum thyroiditis in a heterogeneous American population and used serum antithyroid antibodies to identify postpartum women at risk. Blood was drawn from 1034 consecutive women on their second postpartum day and tested for antimicrosomal and antithyroglobulin antibodies by hemagglutination. Seventy-two women (7.0%) were seropositive for antimicrosomal antibodies, but only seven (0.7%) had antithyroglobulin antibodies. There was a significant difference in the racial prevalence of antimicrosomal antibodies, with seropositivity in 52 of 588 white women (8.8%) versus nine of 367 black women (2.5%; p less than 0.001). Thirty-four of 51 (67%) antimicrosomal seropositive women followed at least 6 months post partum developed biochemical thyroid dysfunction and 20 of these patients required treatment for hypothyroidism. The mean (+/- SEM) serum thyroxine and thyrotropin levels in these patients before treatment were 3.0 +/- 0.3 micrograms/dl (normal 6.1 to 12.3 micrograms/dl) and 77 +/- 17 mU/L (normal 0.3 to 4.0 mU/L), respectively. Psychologic interviews revealed a significant increase in impaired concentration, carelessness, depression, and total complaints when patients with postpartum hypothyroidism were compared with postpartum euthyroid women. Medical evidence now suggests that postpartum thyroiditis is a common event and causes significant symptoms in women who develop hypothyroidism. Therefore, we propose that serum antimicrosomal antibody testing of postpartum women provides a feasible cost-effective screening method of identifying women likely to suffer from this disease.