Somatic-type malignancies (SMs) in patients with testicular germ cell tumors (GCT) are rare and mostly attributed to "transformation" of teratoma, although yolk sac tumor (YST) origin has also been proposed. We studied 124 cases of "SM" of testicular GCT origin from 106 patients to evaluate their morphology, immunohistochemical features (especially the utility of SALL4), and relationship to YST. Primitive neuroectodermal and nephroblastomatous tumors were excluded because of prior studies. Patients ranged in age from 15 to 68 years (mean, 33 y). The tumors ranged from 0.7 to 30 cm (mean, 7.6 cm) and involved the retroperitoneum (64%), abdomen/pelvis (10%), lung (10%), mediastinum (6%), supraclavicular region/neck (4%), testis (4%), and thigh (1%). Most initial diagnoses were sarcomas (n=68) or carcinomas (n=51). On review and immunohistochemical analysis, 7 of 45 adenocarcinomas were reclassified as glandular YSTs (GYST) on the basis of glypican-3 (GPC3) and/or α-fetoprotein positivity and scant/absent reactivity for EMA and CK7. These occasionally (29%) had subnuclear and sometimes supranuclear vacuoles (endometrioid-like), whereas adenocarcinomas were more frequently mucinous (17%) or enteric-type (11%) than endometrioid-like (9%). Both expressed CDX2 frequently (83% and 63%, respectively). MUC protein 2, 4, 5, and 6 expression was more common in adenocarcinomas (7% to 36%) than in GYSTs (0% to 20%) but was infrequent. Both were often positive for SALL4, BerEP4, and MOC31; all were negative for TTF-1. On follow-up (GYST: range, 23 to 169 mo; mean, 81mo; adenocarcinoma: range, 1 to 170 mo; mean, 55 mo), 50% and 33% of patients with GYST and adenocarcinoma, respectively, died of disease. We reclassified 26 of 76 sarcomatoid tumors as sarcomatoid YSTs (SYST) on the basis of positive reactivity for both AE1/3 and GPC3. These tumors often had spindled and epithelioid cells in a fibromyxoid stroma. SYSTs were often (60%) SALL4 positive, whereas sarcomas were all negative. On follow-up (SYST: range, 1 to 259 mo; mean, 62 mo; sarcoma: range, 1 to 327 mo; mean, 70 mo), 50% and 29% of patients with SYST and sarcoma, respectively, died of disease, with most mortality occurring in those with high-grade tumors. We conclude that, on the basis of a panel of immunoreactivities, a significant number of "SMs" in testicular GCT patients are more accurately classified as either GYSTs or SYSTs. Ambiguous glandular tumors should be evaluated for GPC3, α-fetoprotein, CK7, and EMA reactivity and sarcomatoid ones for GPC3, AE1/3, and SALL4 reactivity.