We have investigated, by image analysis, the cell cycle expression of estrogen receptors (ER) on MCF-7 cell line and on MCF-7 xenografts. The results demonstrate, in vitro as well as in vivo, an increase of ER concentration during the G0/G1-phase, followed by a decrease during the S-phase until the late S-phase where a rapid increase was noted. These results confirm that estrogens are involved in the DNA synthesis since ER is expressed in vivo at a maximal level in the late G1. In presence of saturating concentrations of 17 beta-estradiol, the mean ER concentration in G0/G1 phase is significantly decreased compared with the control cells cultured in estrogen-deprived medium. This indicates that 17 beta-estradiol down-regulates ER preferentially in the G0/G1 phase. These data suggest that ER in S and G2/M phases is unable to interact with its ligand. Consequently, estrogens may have no effects on the entry of cells in mitosis. Finally, after long-term tamoxifen treatment of MCF-7 xenografts, a tamoxifen-resistant tumor was developed which was characterized by a change in the profile of ER concentration during the G0/G1 phase. In conclusion, it is possible that the differences in cell cycle distribution of ER could be correlated with different phenotypes of breast cancer and also with different clinical phases of tumoral evolution. However, it remains to be known what is the clinical significance of the ER cell cycle expression in relation to tumor aggressiveness and survival.