CONTEXT: Comorbidities may increase the negative effects of specific anticancer treatments such as androgen suppression therapy (AST). OBJECTIVES: To compare 6 months of AST and radiation therapy (RT) to RT alone and to assess the interaction between level of comorbidity and all-cause mortality. DESIGN, SETTING, AND PATIENTS: At academic and community-based medical centers in Massachusetts, between December 1, 1995, and April 15, 2001, 206 men with localized but unfavorable-risk prostate cancer were randomized to receive RT alone or RT and AST combined. All-cause mortality estimates stratified by randomized treatment group and further stratified in a postrandomization analysis by the Adult Comorbidity Evaluation 27 comorbidity score were compared using a log-rank test. MAIN OUTCOME MEASURE: Time to all-cause mortality. RESULTS: As of January 15, 2007, with a median follow-up of 7.6 (range, 0.5-11.0) years, 74 deaths have occurred. A significant increase in the risk of all-cause mortality (44 vs 30 deaths; hazard ratio [HR], 1.8; 95% confidence interval [CI], 1.1-2.9; P = .01) was observed in men randomized to RT compared with RT and AST. However, the increased risk in all-cause mortality appeared to apply only to men randomized to RT with no or minimal comorbidity (31 vs 11 deaths; HR, 4.2; 95% CI, 2.1-8.5; P < .001). Among men with moderate or severe comorbidity, those randomized to RT alone vs RT and AST did not have an increased risk of all-cause mortality (13 vs 19 deaths; HR, 0.54; 95% CI, 0.27-1.10; P = .08). CONCLUSIONS: The addition of 6 months of AST to RT resulted in increased overall survival in men with localized but unfavorable-risk prostate cancer. This result may pertain only to men without moderate or severe comorbidity, but this requires further assessment in a clinical trial specifically designed to assess this interaction. TRIAL REGISTRATION: clinicaltrials.gov Identifier: NCT00116220.