Cell proliferation, apoptosis, and the expression of Bcl-2 and Bax were investigated in breast tissue of healthy premenopausal women in order to study the effect of the menstrual cycle and reproductive history on the cell turnover in the non-lactating mammary gland epithelium. Immunohistochemistry was used to detect the proliferation-associated antigen Ki-67, as well as Bcl-2 and Bax. Apoptotic cells were identified by enzymatic labelling of fragmentized DNA (TUNEL-technique) and morphologic analysis. Consistent with published data, the proliferative activity and the frequency of apoptotic events as detected by morphologic analysis was higher in the luteal than in the follicular phase of the menstrual cycle. Parity, lactation, and age correlated with lower proliferative activity, whereas the frequency of apoptosis was not significantly influenced by the reproductive history. Staining patterns for Bax and Bcl-2 showed characteristic changes due to the menstrual cycle with a maximum of immunoreactivity for Bcl-2 in the follicular phase and for Bax in the luteal phase. However, there was no statistically significant association between Bcl-2/Bax immunoreactivity and menstrual cycle or reproductive parameters. We conclude that other molecular pathways than the Bax/Bcl-2 antagonism may additionally be involved in the regulation of apoptotic cell death in the breast epithelium. Knowledge of the entire complexity of apoptosis regulation is necessary to understand the observed effects of parity and lactation on mammary epithelial biology, and possibly to be able to influence pathological processes caused by an imbalance between cell renewal and elimination.