This study was undertaken to evaluate the incidence of thyroid autoantibodies in women with a history of recurrent (three or more consecutive) spontaneous abortions. A total of 22 euthyroid non-pregnant habitual aborters were analysed for thyreoglobulin and thyroid peroxidase antibodies; 22 nulligravidae and 22 multigravidae without endocrine dysfunction served as controls. Both thyroid antibodies were assayed using highly sensitive, commercially available enzyme-linked immunosorbent assay kits. Eight of the 22 women with recurrent spontaneous abortions (36%) but only two of the 22 nulligravidae controls (9%; chi 2 test, P = 0.03) and one of 22 multigravidae subjects (5%; chi 2 test, P < 0.01) demonstrated positive titres (> 100 IU/ml, as recommended by the manufacturer) of thyreoglobulin, thyroid peroxidase or both antibodies. The mean +/- SD antibody concentrations were 102.5 +/- 226.5 in the study versus 20.9 +/- 68.8 in the nulligravidae (U-test, P = 0.057) and 26.4 +/- 60.2 IU/ml (P = 0.097) in the multigravidae population for thyroid peroxidase, and 47.7 +/- 57.9 versus 24.1 +/- 31.9 (U-test, P = 0.051) and 28.1 +/- 27.9 IU/ml (P = 0.059) for thyreoglobulin. In conclusion, the incidence of thyroid antibodies in euthyroid women with habitual abortions appears to be significantly increased compared with the controls of reproductive age without previous abortions. Although the important issue of cause and effect has not been fully clarified, this finding suggests that thyroid antibodies may be a marker for autoimmune-mediated recurrent spontaneous abortions, not dissimilar to, for example, anticardiolipin.