This was the first report of the clinical use of nitric oxide donors in the treatment of prostate cancer. The role of nitric oxide in malignant progression has been a subject of controversy, with studies showing either tumor-promoting or tumor-inhibitory roles. These apparently contradictory effects of NO may be explained by the fact that this molecule can regulate phenotypes through a variety of mechanisms depending on local concentrations and the redox state of the cell. Based on our previous findings, the investigators propose that the observed effect of GTN on the PSA of this patient cohort is related to the 'low concentration' effects of NO.