Background: Serum prostate-specific antigen (PSA) has proven to be the most useful tumor marker for prostate cancer. We previously reported the decline rate of PSA levels in patients with cancer of the prostate during radiotherapy; however, on the basis of a small number of patients, no definite conclusions were drawn. Material and Methods: In this report, we update our findings from a prospective study in which changes in PSA levels during radiotherapy (RT) were determined. Pre-RT and weekly PSA levels during 7-8 weeks of RT were measured in 76 stage A-C patients with prostate cancer. Results: In a majority (78%) of patients, the PSA levels declined. The mean rate of PSA decline was 1.3% /day or about 9%/week during RT. 13 patients received hormonal therapy prior to RT; their mean rate of decline (1.8%/day) was not significantly different from the average rate in 63 patients who had no hormonal treatment (1.2%/day). 11 patients had one or more digital rectal examinations (DRE) during the RT course; the mean rate of PSA decline (0.5%/day) was significantly less than the mean rate of decline in 65 patients who had no DRE (1.5%/day; p = 0.033); however, the median rates of decline between these groups were not statistically different. There was a significant (p = 0.0004) transient increase in the PSA values 24 h after the first RT fraction. No significant association was found between rate of decline and age, stage, grade, or race in the group of patients with no hormonal therapy. Tumor volume was calculated from prostate volume and initial PSA level prior to RT; a highly significant correlation was found between tumor volume and the rate of decline (p < O.001). In a multivariate model which included age, stage, grade, race and tumor volume, the latter remained significantly associated (p < 0.001) with the rate of PSA decline. A similar univariate and multivariate analysis showed that PSA density also was associated with the rate of PSA decline during RT (p < 0.001). Conclusion: The rate of PSA decline during RT is associated with the calculated tumor volume; PSA density is also similarly correlated with the rate of PSA decline.