Apoptosis in the human endometrium up to now has been detected during the mid to late luteal phase and therefore connected to the onset of the menstrual shedding. However, there is increasing evidence that regulated apoptosis may be important during decidualization and implantation. To investigate a possible role for apoptosis in the human endometrium and its regulation, we correlated the immunolocalization of the apoptosis regulatory protein bcl-2 and the proliferation marker Ki67 to the in-situ nuclear DNA fragmentation - a key feature of apoptosis - detected by using the terminal deoxynucleotidyl transferase-mediated dUTP nick-end-labelling (TUNEL) method during the menstrual cycle. Whereas proliferation and bcl-2-expression were predominantly detected in the glandular compartment during the proliferative phase, only single apoptotic cells could be shown during this period. During the transformation of the endometrium (days 15-19) proliferation and bcl-2 expression decreased markedly and there was no sign of apoptosis. At the beginning of the implantation window (days 19-20) we could detect the first signs of apoptosis in the glandular epithelia in the basalis, which extended to the functionalis during the luteal phase. Proliferation and bcl-2 expression are limited to the stromal compartment comprising the large granular lymphocytes - during this time, and extend in parallel with apoptosis from the basal to the functional layers. Apoptosis therefore may be related to the loss of the protective effect of bcl-2 and may have significance for the establishment of an endometrium adequately prepared for successful implantation.