Background Epidemiological studies have shown that lycopene has anti-prostate cancer effect. In vitro tests also confirmed that it can promote apoptosis of prostate cancer cells. We investigated the effect of whole-tomato supplement lycopene on the prostate-specific antigen velocity in selected prostate cancer patients. Methods Twenty selected prostate cancer patients were given whole-tomato supplement lycopene 10 mg per day for about 6 months. Blood samples of patients were collected weekly to measure serum prostate-specific antigen (PSA) values. PSA velocity slope, which reflects the change of PSA, and the degree of change were also calculated. By comparing the values of average PSA velocity slope (rise or fall of PSA) before and after the administration of lycopene, the effect of lycopene can be evaluated. Blood chemistry analysis was regular followed as safety control. Results Three patients in the research group withdrew within 3 weeks because of inability to conform. The rest 17 patients continued for an average period of 6 months. Two patients withdrew because of cancer progression (PSA rise) who later received active treatment. The average fall in PSA was equivalent to 2.56% over (i.e. an average slope/d of -0.000 28) the first 3 months. In the last 3 months, average fall in PSA was equivalent to 31.58% (i.e. an average slope/d of -0.003 51). The Wilcoxon rank-sum test showed a statistically significant decrease of PSA velocity slope overall (P=0.000 9). Analysis of the PSA doubling time (pre- vs. post-treatment) showed a median increase over 3 months but this was not statistically significant (P=0.21). No toxic side effect was observed during the whole process. The results indicate that the average PSA change is "decline" in patients, and the degree of the decline is accelerated. Conclusion Administration of lycopene was able to reduce PSA velocity in this study group.