Curcumin I (Cur I) and curcumin III (Cur III) are the yellow coloring phenolic compounds isolated from the spice turmeric. The effect of curcumins on different stages of development of cancer was studied. Cur I inhibited benzopyrene- (BP) induced forestomach tumors in female Swiss mice, and Cur III inhibited dimethylbenzanthracene- (DMBA) induced skin tumors in Swiss bald mice. Cur I also inhibited DMBA-initiated, tetradeconyl phorbol acetate-promoted skin tumors in female Swiss mice. In vitro 3H-BP-DNA interaction studies, and in vivo carcinogen metabolizing enzyme studies revealed that curcumins exert anticarcinogenic activity by altering the activation and/or detoxification process of carcinogen metabolism. Cur I and Cur III also exhibit in vitro cytotoxicity against human chronic myeloid leukemia, which is dose dependent. This study shows that curcumins inhibit cancer at initiation, promotion and progression stages of development.