In a study aimed at evaluating risk factors for carcinoma in situ (CIS) in the contralateral testis of selected patients with testicular cancer, information from 89 patients who have had contralateral testicular biopsies is available. The biopsies were performed because of the coexistence of the previously established risk factors - atrophy of the remaining testis, or a history of maldescent. CIS was diagnosed in 13% of patients, the prevalence being the same for both open and needle biopsies. Compared to its overall prevalence in the contralateral testis in testicular cancer an increased risk of CIS has been found in clinically atrophic testes but has not been found for maldescent. Patients found to have CIS were younger than those in whom it was not detected.