Heat shock proteins of 72 kDa and 90 kDa (HSP70, HSP90) have been suggested to be involved in the functional modulation of sex steroid receptors. We examined the immunohistochemical localization of HSP70 and HSP90 in both the functionalis and basalis layers of normal human endometrium during various phases of the menstrual cycle. Topological correlation with localization of estrogen receptors (ER), progesterone receptors (PR), and proliferation-related Ki-67 antigen was also analyzed. HSP70 was expressed in glandular cells of the basalis but not in the cells of the functionalis during the proliferative phase. In the secretory phase, however, glandular cells of both the basalis and functionalis markedly expressed HSP70. Endometrial stromal cells at the basal layer were positive for HSP70, whereas those cells in the functional layer were negative for HSP70 throughout the menstrual cycle. The topological expression of HSP70 in glandular and stromal cells of the basalis was inversely related to Ki-67 localization. Overexpression of HSP70 in the secretory glands was associated with down-regulation of ER and PR. These findings suggest that HSP70 expression is related to either hormonal regulation of cell proliferation and/or down-regulation of sex steroid receptors. HSP90 was strongly expressed in both glandular and stromal cells during the proliferative phase of the menstrual cycle; in the secretory phase, HSP90 expression was weak in both types of the cells. However, no topological difference in HSP90 expression between the basalis and the functionalis was observed.