Background: The aim was to investigate the use of single agent carboplatin in patients with seminoma stage IIA/B. Patients and methods: In a prospective phase II trial, single agent carboplatin at a dose of AUC 7 mg·min/ml every 4 weeks for three cycles in stage IIA (n = 51) or four cycles in stage IIB (n = 57) was given to 108 patients with previously untreated seminoma stage IIA/B. Patients with residual masses of 3 cm were scheduled to receive secondary surgery. Results: A complete response (CR) was achieved by 88/108 (81%) patients, 17/108 (16%) achieved a partial response (PR), two of 108 (2%) showed no change, and one patient progressed. In all patients with PR the residual disease was 3 cm; yet in two of 17 patients with PR, in two of two patients with NC and in one patient with disease progression residual tumor resection was performed demonstrating vital seminoma. Toxicity was acceptable with grades 3 and 4 myelosuppression, nausea and vomiting in less than 10% of patients each. After a median follow-up of 28 months (range 1–68 months) 14/108 (13%) patients relapsed, all after having achieved a CR. All relapses occurred in the retroperitoneum. One patient died from an unrelated cause. The overall failure rate was 19/108 patients (18%). The overall and disease specific survival was 99% and 100%, respectively. Conclusions: Four cycles of single agent carboplatin AUC 7 do not safely eradicate retroperitoneal metastases in patients with stage IIA/B seminoma.