Curcumin, one of the major components of tumeric, the dried rhizome of Curcuma longa L, has been shown to have anti-proliferating and anti-carcinogenic properties. In this study, we examined the effects of curcumin on cell growth and telomerase activity in human cancer cell lines Bel7402, HL60 and SGC7901. Curcumin (1-32 microM) showed anti-proliferating effects on these cell lines in a dose-dependent manner in vitro, and anti-tumor effects when curcumin (50-200 mg/kg) was orally administered to nude mice transplanted with the cancer cells. When the cells were treated with 1 microM of curcumin for 120 h, apoptotic cells were observed by means of the adridine orange/ethidium bromide staining method, single cell microgel electrophoresis and flow cytometric analysis. On the other hand, suppression of telomerase activity in extracts of the cells treated with 1 microM of curcumin was observed by means of a telomeric repeat amplification protocol - silver staining assay. These results suggest that curcumin could suppress telomerase activity in the cancer cell lines and that the decrease of telomerase expression followed by induction of apoptosis might be involved in the anti-proliferating effect of curcumin.