The objective of this study was to show or to exclude an effect of dietary supplement on rising prostate-specific antigen (PSA) levels. We have studied the effect of a dietary supplement (verum, administered for 6 weeks) containing plant estrogens, antioxidants, including carotenoids, selenium and other putative prostate cancer inhibiting substances in a randomized placebo-controlled double-blind crossover study in 37 hormonally untreated men with prostate cancer and increasing PSA levels. Outcome measures were changes in the rates of change of serum concentrations of total and free PSA and changes in male sex hormone levels. Male sex hormone levels were significantly lower during the verum phase (DHT: -0.11 nmol/L, p = 0.005; testosterone: -1 nmol/L, p = 0.02). Total PSA doubling time was unaffected. Free PSA, which increased during the placebo phase (average doubling time of 68 weeks), decreased during the verum period (average half-life of 13 weeks; p = 0.02). In those men in whom the free androgen index decreased (21 out of 32), a significant decrease in the slopes of both total and free PSA was observed (p = 0.04). Overall total PSA doubling times did not increase significantly during verum. However, the study demonstrates that this dietary intervention reduces DHT and testosterone levels and increases free PSA doubling time (and total PSA doubling time in a relevant subgroup). If future studies confirm that these observations translate into a slowing of disease progression, a dietary intervention may become an attractive option for prostate cancer treatment and prevention.