Differentiating testicular germ cell tumors from sex-cord stromal tumors can be difficult in certain cases because of overlapping morphologic features and/or an absence of clinically apparent hormonal symptoms. Immunohistochemistry may be needed as an ancillary diagnostic tool in this differential diagnostic setting. Steroidogenic factor-1 (SF-1) is a nuclear transcription factor controlling steroidogenesis and is expressed in developing Sertoli and Leydig cells. Although 1 recent study has reported SF-1 nuclear immunoreactivity in testicular sex-cord stromal tumors, the specificity for this marker in germ cell tumors has not been evaluated. After encountering several problematic cases (including some on testicular biopsy), we sought to determine the diagnostic specificity of SF-1 in a large series of germ cell tumors. Nuclear immunohistochemical expression of SF-1 was evaluated in 127 germ cell tumors using tissue microarray technology with 23 non–germ cell tumor tissues as positive internal controls. No nuclear SF-1 expression was identified in any of the 127 germ cell tumors [including choriocarcinoma (3), embryonal carcinoma (25), epidermal inclusion cyst (1), intratubular germ cell neoplasia unclassified (4), seminoma (72), spermatocytic seminoma (2), teratoma (8), and yolk sac tumor (12)]. All 23 non–germ cell tumor tissues showed strong nuclear SF-1 expression in Sertoli and/or Leydig cells [including testicular atrophy (10), cryptorchidism (2), normal testis (4), hypospermatogenesis (1), immature testis (1), intratubular large cell calcifying Sertoli cell tumor (1), Leydig cell tumor (3), and Sertoli only (1)]. This study documents the absence of SF-1 expression in testicular germ cell tumors and supports its specificity for sex-cord stromal lesions in this diagnostic context.