327 Background: To report long-term outcomes of patients (pts) treated with radiotherapy (RT) for stage II testicular seminoma. Methods: Between 1974 and 2007, 52 pts received RT for clinical stage II testicular seminoma after radical inguinal orchiectomy; 3 pts were excluded due to no clinical follow-up. CT staging was used for 46 pts (94%). Estimates of overall (OS), relapse-free (RFS), and cause-specific (CSS) survival were determined using the Kaplan-Meier technique. Major cardiac event (MCE) was defined as myocardial infarction, coronary artery bypass grafting or stenting, or valve replacement. Second malignancy (SM) was defined as biopsy-confirmed malignancy occurring in the RT field. Results: The median pt age was 35 years. AJCC stage was IIA (n=23), IIB (n=7), IIC (n=15), and unknown (n=4). Three pts were treated for paraortic recurrence during surveillance for stage I seminoma. Four pts with IIB (n=1) or IIC (n=3) disease were treated with chemotherapy in addition to RT. The median total RT dose was 30.4 Gy. Prophylactic mediastinal/supraclavicular (MSCV) RT was given to 24 pts (49%). The median follow-up was 18 years (range 0.4-37). Estimates of OS, RFS, and CSS at 10 and 20 years were 94% and 81%, 79% and 70%, and 96% and 96%, respectively. OS, RFS, and CSS were not significantly different between stage groups. Recurrence occurred in 9 pts (18%); sites were MSCV (n=6), para-aortic (n=1), lung (n=1), and peritoneal cavity (n=1). Seven pts were successfully salvaged, while 2 pts died of seminoma. No patient with stage IIA/B that received prophylactic MSCV RT (n=11) experienced MSCV relapse. Among patients that did not receive prophylactic MSCV RT, 2 of 13 (15%) with stage IIA and 3 of 6 (50%) with stage IIB experienced MSCV relapse. MCE occurred in 10 pts (20%) at a median of 18 years (range 7-30) after RT. SM occurred in 5 pts (10%) at a median of 27 years (range 20-34) after RT. Conclusions: Infradiaphragmatic RT alone was associated with a significant risk of MSCV failure, particularly in patients with stage IIB disease, suggesting that chemotherapy may be the optimal treatment in this patient cohort. Most MCE and SM events occurred more than 20 years after RT, highlighting the importance of vigilant long-term follow-up.