BackgroundMen diagnosed with prostate cancer (PC) have an increased risk of depression; however, it is unclear to what extent depression affects long-term survival. A better understanding of such effects is needed to improve long-term care and outcomes for men with PC. ObjectiveTo determine the associations between major depression and mortality in a national cohort of men with PC. Design, setting, and participantsA national cohort study was conducted of all 180 189 men diagnosed with PC in Sweden during 1998–2017. Subsequent diagnoses of major depression were ascertained from nationwide outpatient and inpatient records through 2018. Outcome measurements and statistical analysisDeaths were identified from nationwide records through 2018. Cox regression was used to compute hazard ratios (HRs) for all-cause mortality associated with major depression, adjusting for sociodemographic factors and comorbidities. Subanalyses assessed differences by PC treatment during 2005–2017. PC-specific mortality was examined using competing risks models. Results and limitationsIn 1.3 million person-years of follow-up, 16 134 (9%) men with PC were diagnosed with major depression and 65 643 (36%) men died. After adjusting for sociodemographic factors and comorbidities, major depression was associated with significantly higher all-cause mortality in men with high-risk PC (HR, 1.50; 95% confidence interval [CI], 1.44–1.55) or low- or intermediate-risk PC (1.64; 1.56–1.71). These risks were elevated regardless of PC treatment or age at PC diagnosis, except for youngest men (<55 yr) in whom the risks were nonsignificant. Major depression was also associated with increased PC-specific mortality in men with either high-risk PC (HR, 1.35; 95% CI, 1.28–1.43) or low- or intermediate-risk PC (1.42; 1.27–1.59). This study was limited to Sweden and will need replication in other countries when feasible. ConclusionsIn this national cohort of men with PC, major depression was associated with ∼50% higher all-cause mortality. Men with PC need timely detection and treatment of depression to support their long-term outcomes and survival. Patient summaryIn this report, we examined the effects of depression on survival in men with prostate cancer. We found that among all men with prostate cancer, those who developed depression had a 50% higher risk of dying than those without depression. Men with prostate cancer need close monitoring for the detection and treatment of depression to improve their long-term health outcomes.