Testicular tumors are bilateral in approximately 5% of the cases, although the incidence of bilaterality is affected by the treatment of the primary tumor. A biopsy of the contralateral testis of testicular tumor patients will show carcinoma in situ (CIS) in 5% to 6% of cases. Virtually all untreated cases of CIS will progress into invasive testicular germ cell tumors. In some countries, most testicular tumor patients have a contralateral biopsy at the time of the primary orchiectomy; but in many countries, urologists are not always aware of the advantages of simultaneous contralateral biopsy. The policy in the United States does not encourage surgeons to perform a contralateral biopsy at the initial orchidectomy or to offer a biopsy to patients after referral to a specialist center. The diagnosis of CIS is not of mere academic interest, but makes it possible to offer the patient optimal treatment.