PURPOSE: The purpose of this study was to determine differential points between benign and malignants acrococcygeal germ cell tumors (SGCT) on MR and CT imaging. MATERIALS AND METHODS: MR (n=13) and CT images (n=9)of 19 patients with surgically-proven SGCT (12 benign, 7 malignant) were retrospectively reviewed with regard totumor features and associated findings, as well as to clinical features including age at presentation and ratio of the sexes. Tumor features in cluded location, size, components (ratio of the enhancing solid portion, presence of fatty components, calcification and necrosis), signal intensity and the presence of an encircling low-signal rimon MR. Associated findings included the presence of sacrococcygeal bony defect and effects of the tumor on adjacent structures (anterior displacement of the bladder and rectum, local invasion). RESULTS: Benign SGCT showed early presentation (mean age, 4.7months), a predominantly (over half total volume) external location(58%)and a cystic composition(100%) and the presence of encircling low-signal rim on MR(80%). Malignant SGCT showed late presentation beyond infancy(mean age, 21.7months), a predominantly internal location(86%) and a enhancing solid composition(86%), the presence of tumoral necrosis(71%) and local invasion of adjacent structures(71%). The significant(p<.05) differential points were age at presentation, location, ratio of enhancing solid portions, the presence of tumoral necrosis, the presence of an encircling low-signal rim on MR and local invasion. CONCLUSION: There are certain differential points between benign and malignant SGCT on MR and CT imaging, and an appreciation of these would be helpful for the planning of treatment.