By employing an ammonium sulphate precipitation technique, previous studies have shown circulating immune complexes (cIC) in an increased proportion of women using oral contraceptives, as compared with non-users. The pathogenicity of cIC is well established, as deposition in organs may lead to activation of the complement system and thereby inflammation and tissue destruction. The purpose of the present study was to determine whether estrogen treatment could induce the formation of cIC, as measured by more specific immune complex assays. To test for complement activation, plasma samples were analysed for the complement split product C3d. Thirty-one women treated with estrogen for menopausal hormone deficiency symptoms and 38 untreated controls were analysed for cIC. Using a solid-phase Clq-anti-lgG binding assay, the mean level of clC in the estrogen-treated group did not differ significantly from that of the untreated control group (p>0.05). In a solid-phase Clq-protein A binding assay a very small average increase in the clC values for the estrogen–treated group in comparison with the untreated group was found, expressing a significant difference (p = 0.04). Moreover, the C3d levels for the estrogen–treated group did not differ from the untreated control group. Finally, no differences could be demonstrated in cIC and C3d levels between patients treated with synthetic estrogens and patients treated with natural estrogens.