There is a growing body of interest on the role of beta-carotene and other carotenoids in human chronic diseases, including cancer. While epidemiological evidence shows that people who ingest more dietary carotenoids exhibit a reduced risk for cancer, results from intervention trials indicate that supplemental beta-carotene enhances lung cancer incidence and mortality among smokers. A possible mechanism which can explain the dual role of beta-carotene as both a beneficial and a harmful agent in cancer as well as in other chronic diseases is its ability in modulating intracellular redox status. beta-Carotene may serve as an antioxidant or as a prooxidant, depending on its intrinsic properties as well as on the redox potential of the biological environment in which it acts. This review summarizes the available evidence for a prooxidant activity of beta-carotene in cultured cells, focusing on biochemical and molecular markers of oxidative stress, which have been reported to be enhanced by the carotenoid.