The classification of intratubular germ cell neoplasia of the testis includes an unclassified type (IGCNU), in addition to various other intratubular lesions that show specific forms of differentiation, such as intratubular seminoma and intratubular embryonal carcinoma. Although IGCNU is recognized as a precursor lesion for testicular germ cell tumors, the relationship between differentiated types of intratubular germ cell neoplasia and invasive germ cell tumors of the testis is not well established. The aim of the present study was to examine the association between invasive testicular germ cell tumors and intratubular neoplastic lesions, with particular emphasis on differentiated types of intratubular germ cell neoplasia. The seminiferous tubules adjacent to 42 testicular germ cell tumors were evaluated for the presence of various forms of intratubular germ cell neoplasia. IGCNU was observed in 37 (88%) of 42 cases, whereas intratubular seminoma and intratubular embryonal carcinoma were seen in 19% and 7% of the cases, respectively. Intratubular seminoma was associated primarily with seminomas or mixed germ cell tumors with a seminomatous component, but was also present in a case of a nonseminomatous germ cell tumor. Intratubular embryonal carcinoma was associated exclusively with nonseminomatous germ cell tumors. All cases of intratubular embryonal carcinoma were identified morphologically and exhibited histologic features corresponding to traditional definitions of this lesion. No examples of intratubular embryonal carcinoma as defined by CD30 expression alone in the absence of an intratubular proliferation were observed. The presence of intratubular seminoma in a nonseminomatous germ cell tumor suggests that it is a true preinvasive lesion rather than a manifestation of intratubular spread of an established invasive seminoma. The low incidence of intratubular embryonal carcinoma supports the theory that most nonseminomatous germ cell tumors evolve initially as seminomas, rather than directly from a differentiated intratubular neoplastic lesion.