The purpose of this study was to investigate whether the endometrium of women with unexplained infertility differs in some immunological aspects from the endometrium of normal fertile women. Endometrial biopsies were obtained from 24 normal fertile women (group I) and 24 women suffering from unexplained infertility (group II) at 4, 7, 10 and 13 days following the luteinizing hormone (LH) surge. Endometrial granulated lymphocytes were assessed morphometrically in 2 microns resin sections. A panel of 11 monoclonal antibodies was employed to characterize the leukocyte subsets in frozen sections. Semi-quantification was performed with a Quantimet 970 image analyser. Data were analysed using one- and two-way analysis of variance. Compared with fertile controls, women with unexplained infertility had significantly lower numbers of CD8+ (T suppressor/cytotoxic) cells at each post-LH date. In contrast, the number of CD4+ (T helper/inducer) cells was significantly higher in group II. Throughout the luteal phase, infertile women had fewer CD56+ cells than normal fertile controls. The volume fraction of endometrium occupied by the nuclei of endometrial granulated lymphocytes did not alter with the cycle stage but the mean nuclear diameter and axial ratio decreased from LH+7 to LH+13. The differences observed in endometrial leukocytic subpopulations between fertile and infertile women may contribute to unexplained infertility probably by affecting the embryonic maternal dialogue during the implantation and early placentation period.