Carcinoma in situ (CIS) of the testis is considered to be a precursor of germ cell cancer. Diagnosis is made by the conventional biopsy technique. Only for patients at risk is a screening biopsy justifiable. This group includes patients with testicular cancer in whom the incidence of contralateral second germ cell tumors is increased. In a prospective study we found three cases of testicular CIS in biopsies from the contralateral testes of 61 such patients. All cases with a diagnosis of CIS presented with testicular atrophy (volume less than 12 ml), associated with necrozoospermia in one patient and with azoospermia in two patients. Treatment consisted in local irradiation (20 Gy) of the remaining testis to preserve Leydig cell function. In control biopsies no evidence of CIS or germ cells was found. More than 3 months after therapy, plasma testosterone levels were normal and LH and FSH levels were increased. None of the patients with negative biopsy (n = 49) who were followed up was found to have a second cancer of the contralateral testis. The average observation time so far is 17.2 months.