Epidemiological studies have shown an inverse relationship between serum lycopene levels and the risk of prostate cancer. The objective of this study was to measure the effect of lycopene on the proliferation of LNCaP human prostate cancer cells in culture. A new, water-dispersible lycopene in an appropriate vehicle was used. The stock solution was diluted in the medium to obtain lycopene concentrations of 10(-6), 10(-5), and 10(-4) M; their corresponding vehicles were similarly diluted to be used as controls. Cells were grown for 48 hours in RPMI-1640 medium supplemented with 10% fetal bovine serum and antibiotics. Lycopene was then added at different concentrations, and the cells were allowed to grow for 24, 48, 72, and 96 hours. Lycopene at concentrations of 10(-6) and 10(-5) M significantly reduced the growth of LNCaP cells after 48, 72, and 96 hours of incubation, by 24.4% to 42.8% (P <.05). The inhibitory effect of lycopene was significantly higher than that of the corresponding vehicle controls. In a follow-up experiment, a lower range of lycopene concentrations (10(-9) to 10(-7) M) was used to determine whether there was a dose-response effect. Lycopene significantly decreased the growth of cells in a dose-dependent manner when cells were incubated for 24, 48, 72, or 96 hours (F = 3.150, 11.27, 54.51, and 297.5, respectively; P <.05). The growth inhibitory effect of lycopene on human prostate cancer cells observed in this study suggests a possibly important role for lycopene as an antioxidant in human prostate cancer; however, investigations of other mechanisms are warranted.