A routine biopsy of the contralateral testis obtained during orchiectomy for embryonal carcinoma in a 26-year-old patient was negative for testicular intraepithelial neoplasia (TIN; carcinoma in situ of the testis). However, a rebiopsy that was taken because of unexplained elevation of alpha-fetoprotein 15 months later proved to be positive for TIN. Six previously reported cases of false-negative testicular biopsies obtained during a search for TIN are reviewed. In the light of several thousands of biopsies performed world-wide to date, the number of false-negative biopsies is probably very low. Although TIN is obviously not randomly dispersed throughout the testis in all patients, a routine biopsy of the contralateral testicle in patients with testis cancer remains a valuable tool for early detection of bilateral testicular tumors.-cal distribution of TIN in testes removed for this lesion. Their results suggested that after puberty TIN is usually randomly dispersed throughout the testicle. Support for this concept was recently given by Mumperow et al. (1992). These authors examined tumor-bearing testes and they did not find differences in the presence of TIN in biopsies taken from a location close to the tumor and taken from a location distant from the tumor. Thus, one single biopsy is regard to be representative for the entire testis and one biopsy taken after puberty is also assumed to be reliable for predicting whether the testis will ever develop cancer (Berthelsen and Skakkebaek 1981 a). Conversely, if the biopsy is negative for TIN, a future tumor manifestation in the testicle examined is not expected according to this theory (Skakkebaek et al. 1987). Taken together, the concept of TIN would constitute an ideal avenue for the early detection of testis cancer in high-risk populations with the biopsy being a safe means of discriminating between individuals who will or who will not develop testis cancer.