Curcumin induces cell cycle arrest and apoptosis in human osteosarcoma (HOS) cells. Curcumin is a major component of Curcuma longa rhizome and has been used as a traditional medicine for centuries. In this study, we showed that curcumin induced cell cycle arrest followed by antiproliferation and apoptosis in human osteosarcoma (HOS) cells.Antiproliferative activity was measured with the 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. Nuclear fragmentation was observed with a fluorescence microscope. Flow cytometry was performed to observe cell cycle distribution and apoptotic body appearance. Changes in cell cycle regulatory and apoptosis-related proteins were investigated by Western blot analysis.The IC(50) value of curcumin was approximately 4.0 microg/ml. Induction of apoptosis was evidenced by apoptotic body appearance and chromosomal DNA degradation. Flow-cytometric analysis indicated that curcumin induced successive G(1)/S and G(2)/M phase arrest followed by apoptosis in HOS cells. The G(1)/S and G(2)/S phase arrest was accompanied by down-regulation of cyclin D1, cdc2 and cyclin B1, respectively. Apoptosis was induced by capspase-3 activation and poly(ADP-ribosyl)polymerase (PARP) cleavage.Our results demonstrated that curcumin caused death of HOS cells by blocking cells successively in G(1)/S and G(2)/M phases and activating the caspase-3 pathway.