From 1985 to 1988, 261 unselected patients entered a nationwide Danish study of surveillance only for testicular seminoma stage I. The median follow-up time after orchidectomy was 48 months, range 6-67 months. 49 patients relapsed (19%). Sites of relapse were paraaortic lymph nodes in 41 patients, pelvic lymph nodes in 5, inguinal lymph nodes in 2 and lung metastases in 1 patient. The median time to relapse was 14 months, range 2-37 months. The 4-year relapse-free survival was 80%. 37 of the relapsing patients (76%) had radiotherapy as relapse treatment. Of these patients, 4 (11%) had a second relapse and received chemotherapy. 1 died of disseminated seminoma. Of the relapsing patients, 12 (24%) had chemotherapy as relapse treatment because of bulky (11 patients) or disseminated disease (1 patient). None of these patients have had a second relapse. However, 2 patients died of infection due to chemotherapy-induced neutropenia. Thus, there have been three seminoma-related deaths (1.1%). The testicular tumour size had an independent prognostic significance. The 4-year relapse-free survivals were 94, 82 and 64% for tumours <3, 3 to <6 and > or = 6 cm, respectively. Patients with tumours > or = 6 cm will now be given prophylactic radiation treatment, whereas we will continue to use surveillance only after orchidectomy for patients with tumours <6 cm.