Intratubular germ cell neoplasia is now well established as a precursor to invasive germ cell tumors of the testis. The lesion is usually associated with subsequent development of typical invasive tumors if orchiectomy is not performed. Pathologic findings in intratubular neoplasia consist of germ cell abnormalities within the seminiferous tubules. The atypical cells have large round nuclei, prominent nucleoli, and abundant clear cytoplasm. They have ultrastructural and immunohistochemical characteristics of spermatogenic precursor cells present in the fetal and postnatal testis. Intratubular collections of abnormal germ cells have been found adjacent to invasive germ cell tumors in a high proportion of cases. Intratubular neoplasia occurring in the absence of testicular enlargement has been found in the following situations: cryptorchidism, in testes of some infertile men, dysgenetic testes, and in the contralateral testis following orchiectomy for germ cell cancer. Recognition of intratubular germ cell neoplasia is of special importance because detection of the most common forms of testicular cancer can be accomplished at a preinvasive stage and, therefore, prior to the opportunity for metastasis.