Malignant germ cell tumors may exist as a primary entity in the retroperitoneum. In a CT study of 14 males with this condition (2 seminomas and 12 non-seminomatous tumors) all masses were large, lobulated and of mixed density. Fat plane obliteration against adjacent structures was frequent. The aorta was embedded in 9 patients and the inferior vena cava was affected in 7, 2 of whom had signs of compromised caval blood flow. Distant metastases were found in the lungs (7 patients), liver (n=4), posterior mediastinum (n=3), and in brain and supraclavicular lymph nodes in one patient each. Serum biomarkers were elevated in 11 patients. An extragonadal germ cell tumor should be considered when CT of the abdomen reveals a large retroperitoneal mass with mixed density.