A prospective nonrandomised trial was performed in order to evaluate tumour control and toxicity of low-dose adjuvant radiotherapy in stage I seminoma with treatment portals confined to the para-aortic lymph nodes. Between April 1991 and March 1994, 721 patients were enrolled for the trial by 48 centres in Germany. Patients with pure seminoma and no evidence of lymph node involvement or distant metastases received 26 Gy prophylactic limited para-aortic radiotherapy. Disease-free survival at 5 years was the primary end point. With a median follow-up of 61 months, 675 patients with follow-up investigations were evaluable for this analysis. Kaplan-Meier estimates of disease-free and disease-specific survival were 95.8% (95% CI: 94.2-97.4) and 99.6% (95% CI: 99.2-100%) at 5 years and 94.9% (95% CI: 92.5-97.4%) and 99.6% (95% CI: 99.2-100%) at 8 years, respectively. A total of 26 patients relapsed. All except two were salvaged from relapse. In all, 21 recurrences were located in infradiaphragmatic lymph nodes without any 'in-field' relapse. Nausea and diarrhoea grade 3 were observed in 4.0 and 1.0% of the patients, respectively. Grade 3 late effects have not been observed so far. The results of our trial lend further support to the concept of limited para-aortic irradiation as the recently defined new standard of radiotherapy in stage I seminoma. There is no obvious compromise in disease-specific or disease-free survival compared to more extensive hockey-stick portals, which were used as standard portals at the time this study was initiated.