['We investigated whether the prostate specific antigen nadir predicts prostate cancer specific and all cause mortality in men treated in a randomized trial of radiation with or without 6 months of androgen deprivation therapy.', 'The study included 204 men with cT1b-T2bN0M0 prostate adenocarcinoma and at least 1 unfavorable factor, including prostate specific antigen less than 10 to 40 ng/ml, Gleason 7 or greater, or T3 on magnetic resonance imaging. We performed Fine and Gray regression, and Cox multivariable analysis to determine whether an increasing prostate specific antigen nadir was associated with prostate cancer specific and all cause mortality, adjusting for treatment, age, Adult Comorbidity Evaluation 27 score and cancer prognostic factors.', 'At a 6.9-year median followup median prostate specific antigen nadir was 0.7 ng/ml for radiation alone and 0.1 ng/ml for radiation plus androgen deprivation therapy. The prostate specific antigen nadir (adjusted HR 1.18/ng/ml increase, 95% CI 1.07-1.31, p=0.001) and Gleason 8 or greater (adjusted HR 8.05, 95% CI 1.01-64.05, p=0.049) significantly predicted increased prostate cancer specific mortality. Moderate/severe comorbidity carried a decreased risk (adjusted HR 0.13, 95% CI 0.02-0.96, p=0.045). Higher prostate specific antigen nadir (adjusted HR 1.10/ng/ml increase, 95% CI 1.04-1.17), older age (adjusted HR 1.10/year, 95% CI 1.04-1.15) and interaction between comorbidity score and randomization arm (each p<0.001) increased the all cause mortality risk. Men who achieved a prostate specific antigen nadir of the median value or less had lower estimated prostate cancer specific and all cause mortality at 7 years (3.7% vs 18.3%, p=0.0005 and 31.5% vs 55.0%, p=0.002).'] Does posttreatment prostate specific antigen nadir predict prostate cancer specific and all cause mortality?