The purpose of this investigation was to localize and characterize white blood cell populations in the human ovary through its physiological life cycle. Ovaries from 30 women of reproductive age and from three post-menopausal women were embedded in paraffin or frozen. Clinical information and pathology review were used to obtain accurate menstrual cycle information and to ensure the absence of ovarian disease. Tissue sections were stained for leukocyte phenotypes and the numbers of white blood cells in the ovary were semiquantitatively assessed by two separate examiners using a 0-3 plus (+) scoring system. Our results demonstrated that macrophages and T lymphocytes were the primary immune cells of the ovary, the concentrations of which were dependent on the location and stage of development of the structures containing leukocytes. Developing follicles contained few (+) macrophages located in the theca, while atretic follicles possessed moderate (+2) numbers in the granulosa and few (+) to moderate (+2) numbers in the theca. Newly formed corpora lutea contained few (+) macrophages, while regressing corpora lutea contained abundant (+3) numbers. Human leukocyte antigen (HLA)-DR positive cells were located predominantly at sites where macrophages were present. T lymphocytes were generally not present in the developing follicle but focal, small (+) numbers were observed in blood vessels of the theca. Atretic follicles contained few (+) T lymphocytes in the granulosa and few (+) to moderate (+2) numbers in the theca. Few (+) T lymphocytes were present in new corpora lutea, while moderate (+2) to abundant (+3) numbers were present in regressing corpora lutea. T lymphocytes at all sites were UCHL1 positive. The CD4 (T helper) to CD8 (T suppressor) ration in the corpus luteum was 1:1. B-lymphocytes and natural killer cells were generally absent in the pre-menopausal ovary. The post-menopausal ovary, in contrast, only contained few (+) macrophages, T lymphocytes and natural killer cells in the stroma. In conclusion, our results indicate that the human ovary is an immunologically dynamic tissue containing activated macrophages and T lymphocytes which provide an anatomical basis for immunoendocrine interactions within the ovary.