BACKGROUND: The natural estrogen 17 beta-estradiol (E2) functions as a potent tumor promoter during tumorigenic transformation of the mammary gland. From amongst the various pathways of E2 metabolism upregulation of C16 alpha-hydroxylation of E2 has been associated with carcinogenesis. In the present study in vitro and in vivo experiments were performed on estrogen receptor positive human breast cancer MCF-7 cells to examine whether the natural estrogen E2 and its metabolites 16 alpha-hydroxyestrone (16 alpha-OHE1) and 2-hydroxyestrone (2-OHE1)function as modulators of tumor cell growth. METHODS: An anchorage-independent growth assay was used for in vitro study bycounting the number of tri-dimensional colonies formed by MCF-7 cells suspendedin 0.33% agar. In vivo experiments examined the effect of implanting metabolitematerial pellets into female nude mice. RESULTS: In the anchorage-independent growth assay (AIG), continuous 14-day exposure to E2 and to 16 alpha-OHE1 at 200 ng/ml induced a 59.4% and a 105.9% increase (P= 0.001)respectively in the number of colonies of MCF-7 cells. Identical treatment with 2-OHE1, however, failed to increase AIG relative to that seen in the solvent treated control cultures. In the in vivo tumorigenicity assay, treatment of nude mice with 1.5 mg E2 or 16 alpha-OHE1 resulted in a 335.4% and a 384.1% increase (P< 0.0002) in tumor growth, while identical treatment with 2-OHE1 failed to exhibit any increase relative to the control group. CONCLUSION: These results suggest that the 16 alpha- and 2-hydroxylated metabolites of E2 may directly affect in vitro growth of MCF-7 cells via an autocrine mechanism and in vivo growth via paracrine mechanisms. Thus, E2-mediated growth regulation in MCF-7 cells may in part be due to distinct effects of specific E2 metabolites on the breast cancer cells.