Background: Standard treatment options for stage IIA/B seminoma include either extensive paraaortal/pelvic radiotherapy or intensive chemotherapy with 3x BEP or 4x EP. Both treatment modalities are associated with excellent efficacy but also a high rate of acute and late toxicities. Therefore, de-escalation strategies appear warranted for this patient group aiming to minimize acute and long-term toxicities while maintaining efficacy. SAKK 01/10 is a joint project of the Swiss Group for Clinical Cancer Research and the German Testicular Cancer Study Group. Methods: Patients with stage IIA/B seminoma (de novo or relapse on active surveillance) were eligible for participation in this single arm phase II trial. A repeat scan was advised in patients with equivocal lymph node enlargement. Treatment consisted of one cycle carboplatin AUC7 followed by involved-node radiotherapy with 30 Gy in stage IIA and 36 Gy in stage IIB disease. The primary endpoint of the trial is 3-year progression free survival. We report on treatment compliance and early toxicity during treatment and within 30 days. Results: 120 patients with stage IIA/B seminoma were recruited from 10/12 until 06/18 in 20 study centers in Switzerland and Germany. 116 patients were eligible and initiated treatment per protocol (40% stage IIA, 60% stage IIB). All patients received chemotherapy (CT) with a median applied dose of 984 mg (range: 560-1920 mg). The median planning target volume (PTV) for radiotherapy (RT) was 297 cm³ (range: 24-1047 cm³). RT was delayed/interrupted in two patients due to adverse events. During CT, grade 2 and grade 3 adverse events were seen in 21% and 2% of all patients respectively (most common grade 2 events: neutropenia 6%, nausea 5%). During RT, grade 2 and grade 3 adverse events were seen in 26% and 6% of all patients respectively (most common grade 2 events: neutropenia 15%, nausea 6%). One case of transient creatinine increase was reported as a severe adverse event, resolving without sequelae. Conclusions: Treatment with one cycle carboplatin AUC7 and 30-36 Gy involved-node radiotherapy for stage IIA/B seminoma is feasible and demonstrates a very favorable early toxicity profile. Clinical trial information: NCT01593241