156 patients with stage I non-seminomatous testicular germ cell tumour entered a countrywide randomized study comparing the effect of postoperative irradiation of retroperitoneal lymph nodes with surveillance only. A total of 150 patients were included in this investigation of factors associated with increased risk of relapse. Thirty-four patients (23 per cent) had a relapse, i.e. 11/67 (16 per cent) in the former and 23/83 (28 per cent) in the latter group. Histopathological studies of the orchidectomy specimens were performed in order to identify features associated with relapse. Pure embryonal carcinoma and vascular involvement were significantly associated with increased risk of relapse. Thus, the relapse rate in the observation group in patients with pure embryonal carcinomas was 50 per cent (5/10) and 36 per cent (19/53) in all patients with tumours with vascular involvement.