Xanthohumol is one of the main flavonoids in hop extracts and in beer. Very few investigations of xanthohumol have studied hepatocellular carcinoma. In this study, the inhibitory effects of xanthohumol on human hepatocellular carcinoma cell lines were investigated. The IC(50) values of xanthohumol for two hepatocellular carcinoma cell lines and one normal hepatocyte cell line were 108, 166 and 211 microm, respectively. Normal murine hepatocyte cell line had more resistance to xanthohumol than hepatocellular carcinoma cell lines. Besides, the inhibitory effects of xanthohumol on human hepatocellular carcinoma cell lines were attributed to apoptosis as indicated in the results of flow cytometry, fluorescent nuclear staining and electrophoresis of oligonucleosomal DNA fragments. Hop xanthohumol was more efficient in the growth inhibition of hepatocellular carcinoma cell lines than the flavonoids silibinin and naringin from thistle and citrus. It was shown for the first time that xanthohumol from hops effectively inhibits proliferation of human hepatocellular carcinoma cells in vitro.