Pre-treatment with high-dose 1alpha,25-dihydroxyvitamin D(3) (1,25-VD) enhanced the antitumor activity of docetaxel in the androgen-independent prostate cancer cell line, PC-3. The effect manifested as an increasing population of apoptotic cells and amount of pro-apoptotic protein, Bax, under combined treatment compared with single treatment of either 1,25-VD or docetaxel alone. We further demonstrated that pre-treatment with 1,25-VD reduced the expression of multidrug resistance-associated protein-1 at both the mRNA and protein levels. This suggests pre-treatment with 1,25-VD can potentiate cytotoxicity of docetaxel in PC-3 due to 1,25-VD reducing multidrug resistance-associated protein-1 expression.