Chalcones xanthohumol (X) and desmethylxanthohumol (DMX), present in hops (Humulus lupulus L.), and the corresponding flavanones isoxanthohumol (IX, from X), 8-prenylnaringenin (8-PN, from DMX), and 6-prenylnaringenin (6-PN, from DMX), have been examined in vitro for their anti-proliferative activity on human prostate cancer cells PC-3 and DU145. X proved to be the most active compound in inhibiting the growth of the cell lines with IC50 values of 12.3+/-1.1 microM for DU145 and 13.2+/-1.1 microM for PC-3. 6-PN was the second most active growth inhibitor, particularly in PC-3 cells (IC50 of 18.4+/-1.2 microM). 8-PN, a highly potent phytoestrogen, exhibited pronounced anti-proliferative effects on PC-3 and DU145 (IC50 of 33.5+/-1.0 and 43.1+/-1.2 microM, respectively), and IX gave comparable activities (IC50 of 45.2+/-1.1 microM for PC-3 and 47.4+/-1.1 microM for DU145). DMX was the least active compound. It was evidenced for the first time that this family of prenylated flavonoids from hops effectively inhibits proliferation of prostate cancer cells in vitro.