Background: Patients with stage I testicular seminoma can be managed by surveillance or treated with adjuvant radiotherapy or chemotherapy. Concerns of second malignancies induced by radiotherapy have led to a preference for adjuvant chemotherapy. The TE19 trial demonstrated the non-inferiority of a single cycle of carboplatin, given within 8 weeks of orchidectomy, to adjuvant para-aortic radiotherapy in terms of reduction in recurrence rate. This study also suggested a lower relapse rate if GFR estimation was radionuclide based. Methods: A review of case records of germ cell tumor patients within the Anglian Germ Cell Cancer Group between 10/01/2005 and 31/12/2012 was conducted to identify patients with stage I testicular seminoma who received a single cycle of adjuvant carboplatin. Tumor characteristics, clinical management, and patient outcomes were assessed. Results: A total of 336 patients with stage I seminoma who received adjuvant carboplatin were identified. The median age of diagnosis was 38 (range 21 to 78). Median tumor size was 40mm. The rete testis was involved in 131 cases (39%). A radionuclide based estimation of GFR was used in 318 patients (94.6%), the remaining patients GFR was estimated by the Cockcroft-Gault equation. The median time from orchidectomy to receiving chemotherapy was 46 days. Of the 336 patients, 22 relapsed (6.5%), with nine relapsing after 2 years (seven between 24 and 36 months, and wo after 36 months). The median time to relapse was 18 months. There was a trend towards lower progression free survival in patients whose chemotherapy was dosed based on GFR estimation by the Cockcroft-Gault equation. All relapsed patients were successfully treated with combination chemotherapy. Conclusions: These results demonstrate a higher rate of relapse for stage I seminoma patients treated with adjuvant carboplatin compared with the published data from the TE19 study. In addition they confirm that adjuvant carboplatin for treatment of stage I testicular seminoma leads to excellent outcomes in terms of long-term cure. The use of radionuclide based estimation of GFR is advised. Relapses beyond 2 years are not rare and routine cross-sectional imaging beyond this point should be considered.