Objectives: There is no consensus regarding the management of Stage 1 seminomas following inguinal orchiectomy. In this study, we evaluated the treatment results and treatment-related toxicity for patients with Stage 1 seminomas treated with adjuvant radiotherapy (RT) at a single institution. Methods: Sixty-five patients who underwent adjuvant RT following orchiectomy for Stage 1 seminomas between January 1996 and December 2007 were retrospectively reviewed. The age, tumor location, histopathological type, stage, tumor size, RT field, and radiation dose were recorded for all patients. Results: The patients' ages ranged from 17 to 61 years (median, 37 years). Sixty-three patients (97%) were diagnosed with classical seminoma and the remaining two patients (3%) had spermatocytic seminoma. After orchiectomy, 37 patients (57%) received para-aortic RT and 28 patients (43%) received dog-leg field RT. RT was applied with 1.8u2 Gy/day fractionation and the median RT dose was 26 Gy (range, 20u38). Follow-up ranged from 0.3 to 18 years (median, 9.5 years). Local control had been achieved in all patients and all of them were alive with no evidence of disease. Fifty-one patients (77%) had at least 5 years of follow-up and 27 patients (41%) had at least 10 years of follow-up. Overall survival at 10 years was 100%. Conclusion: Although retrospective in nature, this single-institutional study provides useful information about the outcomes and toxicities associated with adjuvant RT in patients with Stage 1 seminomas reporting excellent disease control and survival rates at the expense of acceptable toxicity.