We investigated, using an image analysis system, the immunohistochemical localization of leukocyte subpopulations and human leukocyte antigen (HLA)-DR in 30 normal-cycling human ovaries in order to better understand local immunological events in human ovaries. All subtypes of T lymphocytes examined (CD3+, CD4+ and CD8+ cells) were sporadically observed in the stroma and theca layers of follicles throughout the menstrual cycle (ranging from 4.32 to 6.25 cells/10(-7) m2, 1.67 to 3.33 cells/10(-7) m2 and 2.33 to 3.44 cells/10(-7) m2, respectively for the three subtypes), and subsequently, increased in number in atretic follicles (78.70 +/- 6.90, 31.13 +/- 2.54 and 43.31 +/- 3.35). After ovulation, the number of T lymphocytes was markedly low in the early and mid corpus luteum (13.88 +/- 1.62, 4.18 +/- 0.50 and 6.53 +/- 0.45). The number increased in the late corpus luteum, and was highest in the late degenerating corpus luteum (255.67 +/- 27.10, 102.12 +/- 7.80 and 137.34 +/- 12.50). HLA-DR was sporadically positive in fibroblasts in the stroma and theca layers of follicles (means ranged from 1.25 to 1.82 cells/10(-7) m2), and increased in atretic follicles (24.68 +/- 2.25). HLA-DR+ cells were markedly low in the early and mid corpus luteum (2.16 +/- 0.88), increased in the late corpus luteum, and reached a plateau in the late degenerating corpus luteum (121.84 +/- 17.73). The great majority of these increased HLA-DR+ cells were macrophages. Results of our study suggest that T lymphocytes and/or macrophages play important roles in luteal regression and follicular atresia in normal-cycling human ovaries.