Curcumin (diferuloylmethane), the naturally occurring yellow pigment in turmeric and curry, is isolated from the rhizomes of the plant Curcuma longa Linn. Curcumin inhibits tumorigenesis during both initiation and promotion (post-initiation) periods in several experimental animal models. Topical application of curcumin inhibits benzo[a]pyrene (B[a]P)-mediated formation of DNA-B[a]P adducts in the epidermis. It also reduces 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced increases in skin inflammation, epidermal DNA synthesis, ornithine decarboxylase (ODC) mRNA level, ODC activity, hyperplasia, formation of c-Fos, and c-Jun proteins, hydrogen peroxide, and the oxidized DNA base 5-hydroxymethyl-2'-deoxyuridine (HmdU). Topical application of curcumin inhibits TPA-induced increases in the percent of epidermal cells in synthetic (S) phase of the cell cycle. Curcumin is a strong inhibitor of arachidonic acid-induced edema of mouse ears in vivo and epidermal cyclooxygenase and lipoxygenase activities in vitro. Commercial curcumin isolated from the rhizome of the plant Curcuma longa Linn contains 3 major curcuminoids (approximately 77% curcumin, 17% demethoxycurcumin, and 3% bisdemethoxycurcumin). Commercial curcumin, pure curcumin, and demethoxycurcumin are about equipotent as inhibitors of TPA-induced tumor promotion in mouse skin, whereas bisdemethoxycurcumin is somewhat less active. Topical application of curcumin inhibits tumor initiation by B[a]P and tumor promotion by TPA in mouse skin. Dietary curcumin (commercial grade) inhibits B[a]P-induced forestomach carcinogenesis, N-ethyl-N'-nitro-N-nitrosoguanidine (ENNG)-induced duodenal carcinogenesis, and azoxymethane (AOM)-induced colon carcinogenesis. Dietary curcumin had little or no effect on 4-(methylnitosamino)-1-(3-pyridyl)-1-butanone (NNK)-induced lung carcinogenesis and 7,12-dimethylbenz[a]anthracene (DMBA)-induced breast carcinogenesis in mice. Poor circulating bioavailability of curcumin may account for the lack of lung and breast carcinogenesis inhibition.