OBJECTIVE This study was undertaken to assess baseline endometrial molecular events in the ATAC (Arimidex, tamoxifen, alone, or in combination) trial of breast cancer adjuvant therapy. STUDY DESIGN Estrogen receptor (ER) and progesterone receptor (PR) levels and markers of cell proliferation (Ki67) and apoptosis ( Bcl -2) were assessed in 93 patients at baseline. RESULTS An inactive/atrophic endometrium was found in 63 patients, 5 had a proliferative endometrium, and 12 had a secretory endometrium. Thirteen endometrial polyps were analyzed. Inactive endometrium showed high levels of ER in the glandular epithelium, whereas in more than 50% of samples, PR expression was negative or low (+) in the glandular epithelium, and stroma. Ki67 expression was low in both the glandular epithelium and the stroma of the inactive endometrium, whereas Bcl -2 expression was mostly high or very high (+++/++++) in the glandular epithelium. Bcl -2 was strongly expressed (+++/++++) in the glandular epithelium of polyps. CONCLUSION Although all patients were asymptomatic, some had endometrial pathology.