Bcl-2 is a proto-oncogene initially described in the (14;18) translocation in follicular lymphoma. It has been shown to prolong cell survival by preventing apoptosis. Endometrium undergoes rapid proliferation and differentiation under hormone control and is thus an excellent model to study the hormone dependency of Bcl-2 expression. We studied Bcl-2 expression by an immunohistochemical method in 53 samples of normal endometrium randomly distributed throughout the menstrual cycle, as well as five samples of hyperplastic endometrium. Bcl-2 staining predominated in glandular cells and peaked at the end of the follicular phase. Bcl-2 expression disappeared at the onset of secretory activity. The stroma, surface lining epithelium and arterial vessels also displayed cyclic variations in Bcl-2 expression. These results strongly suggest hormone-dependent regulation of Bcl-2 expression, which could play an important role in tumorigenesis.