Purpose: Positive surgical margins are common after radical prostatectomy, and the role of adjuvant therapy in such cases is controversial. We determined the benefit of postoperative external beam radiation therapy in patients with margin positive prostate cancer with respect to biochemical progression or cancer recurrence. To decrease confounding factors that may affect the likelihood of biochemical progression our study was limited to men with organ confined cancer and a single positive margin. Materials and Methods: We retrospectively evaluated the records of a nested matched cohort of 76 patients with pathological stage T2N0 prostate cancer and a single positive margin who underwent adjuvant radiation therapy within 3 months of radical prostatectomy. There was a positive margin at the prostatic apex in 35 cases, prostatic base in 18, posterior prostate in 11, urethra in 7, and prostatic apex and urethra in 5. These patients were matched 1:1 with 76 controls who did not receive adjuvant radiation therapy. Neither group received androgen deprivation therapy. Patients and controls were matched exactly for the margin positive site, age at surgery, preoperative serum prostate specific antigen, Gleason score and DNA ploidy. Biochemical relapse was defined as posttreatment PSA greater than 0.2 ng./ml. Results: Overall there was significant estimated improvement plus or minus standard error in 5-year clinical and biochemical progression-free survival in 88% ± 5% versus 59% ± 11% of patients treated with adjuvant radiation therapy versus no radiation therapy (p = 0.005). No patient who received radiation therapy had local or distant recurrence, while 16% of controls had recurrence (p = 0.015). When stratified by site of margin positivity, the 5-year estimated clinical and biochemical progression-free rate in 18 cases and controls with a positive base margin was 95% ± 15% and 65% ± 13%, respectively (p = 0.02). The rate in 35 cases and cases with a positive apex margin was 95% ± 5% and 64% ± 15%, respectively (p = 0.07). Limited sample size precluded analysis of the other sites. Conclusions: Patients with localized prostate cancer and a singe positive surgical margin appear to have a lower rate of biochemical relapse at 5 years when adjuvant radiation therapy is administered. Definitive evidence of the beneficial effect of adjuvant radiation therapy for patients with involved surgical margins awaits conclusion of randomized clinical trials.