The authors describe 10 sex cord-stromal tumors of the testis that incorporated germ cells, thereby mimicking the unclassified type of mixed germ cell sex cord-stromal tumor (MGCSCST). These neoplasms occurred in patients from 3 to 48 years old (mean age, 26 years) who presented with testicular masses. On microscopic examination, nine tumors had a combination of tubular and cord-like arrangements of sex cord cells with transition to spindle-shaped tumor cells. They were diagnosed as either unclassified sex cord-stromal tumors (n = 5) or Sertoli-stromal cell tumors (n = 4). One tumor was a pure Sertoli cell tumor. The admixed germ cells were usually at the periphery and in clusters, but occasionally were in the center or more diffuse. In nine patients the germ cells resembled spermatogonia, having round nuclei with uniform, dusty chromatin and inconspicuous or small nucleoli. None of these cells stained with a variety of markers used for neoplastic germ cells, and in one case in which the non-neoplastic Sertoli cells were strongly reactive for inhibin but the neoplastic Sertoli cells were not, all the germ cells within the tumor occurred adjacent to inhibin-positive Sertoli cells. With static cytophotometry, a diploid deoxyribonucleic acid content was found in these germ cells in the two investigated cases. In one case the germ cells had the morphologic appearance of seminoma cells and they stained positively for the markers of neoplastic germ cells. This case was interpreted as a "collision" tumor between a Sertoli cell tumor and a seminoma. The authors conclude that sex cord-stromal tumors with entrapped germ cells of the testis are more common than unclassified MGCSCSTs--a bona fide testicular example of which has not been seen by any of the authors.