In a series of 773 patients with the diagnosis of a testicular germ cell tumour, treated at Hannover University Medical School between 1972 and 1985 and with a median follow-up of 9 years (60-210 months), bilateral testicular tumours occurred in 27 (3.5%) patients. None of 157 patients receiving chemotherapy for metastatic disease of the first tumour developed a metachronous bilateral tumour. Of 24 patients with metachronous tumours 23 had stage I and 1 patient had stage II at the time of initial diagnosis. The second testicular tumour was stage I in 18 patients, stage II in 5 and stage IV in 1 patient. 3 patients (13%) relapsed after treatment for their second germ cell tumour (surveillance 13 patients, radiotherapy 7 patients, lymph node dissection 2 patients and chemotherapy 2 patients), 1 of which died after refusing further treatment. The cure rate was 96% in patients with bilateral disease. Routine biopsy of the contralateral testis to identify existing carcinoma in situ (CIS) is recommended. Patients with CIS must be informed about their increased risk of a second testicular tumour. Irradiation of CIS or close clinical follow-up might both constitute appropriate strategies for the management of these patients.