226 Background: To better understand the impact of RT on mortality, we analyzed long-term survival and patterns of excess mortality in men with stage I seminoma. Methods: 9,045 men with stage I seminoma were identified in the Surveillance Epidemiology and End Results database. Time to testicular-cancer mortality (TCM), death from second malignancy (SM), cardiovascular mortality (CVM) or suicide (SUIC) and all-cause mortality (ACM) were calculated. Survival estimates were calculated using the Kaplan-Meier method. Gender- and age-adjusted standardized mortality ratios (SMR) were calculated using U.S. population data. Cox and Fine and Gray multivariable analysis were used to evaluate the effect of RT on mortality outcomes. Results: 7,025 men (78%) received RT. After a median follow-up of 11.7 years, 869 men (9.6%) had died: sixty-five from TCM, 279 from SM, 169 from CVM and 37 from SUIC. 10-year rates of ACM and TCM were 4.24% and 0.52% among men who received RT and 7.14% and 1.22% among men who did not. Compared to the adjusted general population, men with seminoma had increased risk of ACM (SMR 1.12; 95% confidence interval [CI] 1.12-1.28), SM (SMR 1.78; 95% CI 1.58-2.00) and SUIC (SMR 1.40; 95% CI 1.02-1.94) and decreased risk of CVM (SMR 0.73; 95% CI 0.62-0.84). Rates of ACM, SM and SUIC (SMR, all p < 0.05) were increased whether RT was used or not. Men who received RT were less likely to die (adjusted hazard ratio [AHR] 0.76; 95% CI 0.65-0.89; p < 0.001) and had a lower risk of TCM (AHR 0.39; 95% CI 0.24-0.65; p < 0.001). There was no difference in CVM between men who did and did not receive RT (AHR 0.89; 95% CI 0.60-1.15; p = 0.230) and a numerical increase in SM in men who received RT as compared to others (AHR 1.25; 95% CI 0.90-1.72; p=0.180). Conclusions: Compared to the general population, men with a history of stage I seminoma had increased risks of all-cause mortality, death from second malignancies, and suicide. Our data suggest that 15 years after diagnosis, men who did receive RT may be more likely to die from a second malignancy than men who did not. Although not receiving RT was associated with higher testicular-cancer mortality, the results may reflect decreased access to care or follow-up as active surveillance protocols were not common during the study era. No significant financial relationships to disclose.