NAD(P)H:quinone oxidoreductase (NQO1) has been reported to play an important role in cell death caused by beta-lapachone (beta-lap), 3,4-dihydro-22,2-dimethyl-2H-naphthol[1,22b]pyran-5,6-dione. This study investigated whether cisplatin (cis-diamminedichloroplatinum) sensitizes cancer cells to beta-lap by upregulating NQO1. The cytotoxicity of cisplatin and beta-lap alone or in combination against FSaII fibrosarcoma cells of C3H mice in vitro was determined with a clonogenic survival assay and assessment of gamma-H2AX foci formation, a hallmark of DNA double-strand breaks. The cellular sensitivity to beta-lap progressively increased during the 24 h after cisplatin treatment. The expression and enzymatic activity of NQO1 also increased during the 24 h after cisplatin treatment, and dicoumarol, an inhibitor of NQO1, was found to nullify the cisplatin-induced increase in beta-lap sensitivity. The role of NQO1 in the cell death caused by beta-lap alone or in combination with cisplatin was further elucidated using NQO1-positive and NQO1-negative MDA-MB-231 human breast cancer cells. Cisplatin increased the sensitivity of the NQO1-positive but not the NQO1-negative MDA-MB-231 cells to beta-lap treatment. Combined treatment with cisplatin and beta-lap suppressed the growth of FSaII tumors in the legs of C3H mice in a manner greater than additive. It is concluded that cisplatin markedly increases the sensitivity of cancer to beta-lap in vitro and in vivo by upregulating NQO1.