Nine-hundred-and-one women presenting in an antenatal clinic at the 60th week of pregnancy were tested for antithyroid antibodies. A group of 113 antibody-positive women and 108 antibody-negative age-matched controls were HLA typed and followed prospectively at 6-weekly intervals through pregnancy and for 12 months postpartum. Forty-five of the women developed biochemical evidence of postpartum thyroid dysfunction (PPTD) of whom 36 were antibody positive. Compared with a local control population (n = 600), and using multiplex analysis, there was a significant increase in the combinations HLA B8, DR3 and HLA A1, B8, DR3 from 22.5% to 40.0% (P less than 0.02) and from 18.6% to 35.6% (P less than 0.01) respectively in the women who developed PPTD. The well-recognized association of these haplotypes with other organ-specific autoimmune diseases provides further support for autoimmune events being implicated in the development of PPTD.