Beta-lapachone, the product of a tree Tabebuia avellanedae from South America, is known to exhibit various pharmacologic properties, the mechanisms of which are poorly understood. In the present study, we investigated further possible mechanisms by which beta-lapachone exerts its anti-proliferative action in cultured human prostate carcinoma DU145 cells. Exposure of DU145 cells to beta-lapachone resulted in growth inhibition and induction of apoptosis in a dose-dependent manner as measured by MTT assay, fluorescent microscopy, and flow-cytometry analysis. The increase in apoptosis was associated with a dose-dependent up-regulation in pro-apoptotic Bax expression, down-regulation of anti-apoptotic Bcl-2, and proteolytic activation of caspase-3 protease. We found beta-lapachone decreased the levels of cyclooxygenase (COX)-2 mRNA and protein expression without significant changes in the levels of COX-1, which was correlated with a decrease in prostaglandin E2 (PGE2) synthesis. Furthermore, beta-lapachone treatment markedly inhibited the activity of telomerase in a dose-dependent fashion. Additionally, the expression of human telomerase reverse transcriptase (hTERT), a main determinant of the telomerase enzymatic activity, was progressively down-regulated by beta-lapachone treatment. Taken together, these findings provide important new insights into the possible molecular mechanisms of the anti-cancer activity of beta-lapachone.