Cholangiocarcinoma represents a challenging primary malignancy of the liver with no effective medical therapy and a pour prognosis. We have investigated the role of tamoxifen and estrogen receptors (ERs) in the regulation of growth of human cholangiocarcinoma. Two human cholangiocarcinoma cell lines, OZ and SK-ChA-1, were grown in the presence of graded concentrations of tamoxifen; the effects on cell growth were determined by cell counting or 3-(4,5-dimethylthiazol-2yl)-2,5- diphenyltetrazolium proliferation assay. The presence of ER protein was tested by indirect immunofluorescence and immunoprecipitation. In addition, cells were grown in estrogen-depleted media supplemented with exogenous 17β- estradiol. ER mRNA was evaluated by reverse transcription-PCR and Northern blotting. Finally, one cholangiocarcinoma cell line was grown as a xenograft in athymic nude mice; tamoxifen effects on in vivo tumor growth were determined with biweekly caliper measurements. Tamoxifen (5-10 μM) caused dose-dependent in vitro growth inhibition of two human cholangiocarcinoma cell lines. In addition, growth inhibition of one cell line (SK-ChA-1) grown as a xenograft in nude mice by tamoxifen was observed. The presence of ER protein was suggested by 17β-estradiol stimulation of tumor cell growth in vitro and confirmed by immunoprecipitation. Immunofluorescence microscopy was ineffective at detection of ER protein. Reverse transcription-PCR demonstrated the presence of ER mRNA in both cell lines. Northern blot analysis confirmed the presence of full-length 6.5-kb ER mRNA. No ER deletion mutants were detected. Tamoxifen inhibited the growth of human cholangiocarcinoma in vitro and in vivo. ER protein and mRNA were detected in both cell lines. The mechanism(s) of tamoxifen-mediated growth inhibition is unclear but may occur via ER protein or additional pathways. The ability of tamoxifen to inhibit tumor growth may offer an alternative adjunctive treatment for cholangiocarcinoma.