Tamoxifen (TAM), used to treat estrogen receptor (ER)-positive breast cancer, is a well known estrogen antagonist in the breast, but a partial estrogen agonist in the endometrium. In addition, TAM metabolites, such as 4'-hydroxy-tamoxifen (4-OH-TAM), have been shown to be more potent than the parent compound. The objective of this study was to determine the effects of 4-OH-TAM and estradiol (E2) on two human endometrial adenocarcinoma cell lines, HEC-1B and HEC-1A. When HEC-1B cells were treated with lower concentrations (10-1,000 nM) of 4-OH-TAM or E2 for 1-3 days, no significant difference in the percentage of cell survival was observed among the varying concentrations. At higher 4-OH-TAM or E2 concentrations (1-100 µM), HEC-1B and HEC-1A cells responded similarly with an obvious decrease in cell growth noted at 10 and 100 µM 4-OH-TAM and 100 µM E2. In order to address the observed cell death, DNA laddering was performed at various time intervals with 4-OH-TAM (10 µM) or E2 (10 or 50 µM). DNA gel electrophoresis failed to show the typical laddering pattern (180-200 bp) observed in apoptosis. Furthermore, western blot analysis of caspase-8 and -3 failed to demonstrate caspase activity. These results suggest that apoptosis was not the underlying cellular mechanism of cell death. Due to the lack of apoptotic markers, a cytotoxic (cell death) effect was differentiated from a cytostatic (growth inhibition) effect confirming that cell death had occurred. In summary, micromolar concentrations of 4-OH-TAM induced a non-apoptotic cytotoxic effect in the endometrium; however further studies are needed to elucidate the cytotoxic pathway being utilized.