To discriminate lipomas from atypical lipomatous tumor/well-differentiated liposarcoma (ALT/WDL) we perform fluorescence in situ hybridization (FISH) for MDM2 amplification in several problematic situations: "lipomas" >10 cm, lesions with equivocal atypia, recurrent "lipomas," all retroperitoneal/pelvic/abdominal "lipomas", and in cases not fitting the above criteria but having worrisome clinical or radiologic features. To ascertain the validity of these criteria, we have reviewed our experience with 301 consecutive differentiated lipomatous tumors in which the diagnosis of ALT could not be established on the basis of histologic sections and in which FISH was performed on the basis of the above criteria. The final diagnosis was based on MDM2 amplification status. Given the nature of this study to evaluate difficult lesions, most cases included (74%) were received in consultation. This enhanced our study series for borderline cases, and the data presented may not be generalizable to adipocytic tumors seen outside a subspecialty setting. Of 301 cases, 108 proved to be ALT/WDL (36%). The most common test indication was size >10 cm (n=187), followed by equivocal atypia (n=145), retroperitoneal/pelvic/abdominal location (n=86), recurrence (n=33), and clinical concern (n=12). Of the tumors >10 cm, 68 (36%) proved to be ALT/WDL, whereas the remainder were interpreted as lipoma or its variants (eg, spindle cell or pleomorphic lipoma). The 2 groups did not differ statistically in size, although ALTs consistently occurred in patients above 50 years of age. Of the cases with equivocal atypia, 72 (50%) proved to be ALT/WDL. Those in the retroperitoneum/abdomen/pelvis were ALT/WDL in 30 cases (35%), and those that had recurred were ALT in 18 cases (55%). Recurrence, atypia, and having multiple indications for testing were more common in ALT than in benign lesions (P=0.02, 0.0001, 0.0012, respectively). No ALT/WDL occurred in the hands and feet, and only a single ALT/WDL was superficial (1 ALT/WDL vs. 60 lipoma/spindle cell or pleomorphic lipoma). Small (<10 cm) retroperitoneal tumors without additional features were always benign (n=9). On the basis of our results, FISH testing is recommended for: (1) recurrent lesions; (2) deep extremity lesions that are >10 cm in patients over 50 years of age; (3) in cases with equivocal atypia; (4) in lesions of the retroperitoneum/pelvis/abdomen, and in special clinical situations as directed by treating clinicians. Testing is low yield in superficial lesions, in small extremity lesions without additional indicators for testing, in large extremity lesions without additional features in patients under the age of 50, and in lesions arising in the hands/feet. More evidence is needed regarding testing in small retroperitoneal lesions without additional features. By adopting these criteria, we could have avoided testing 74 cases, missing a single superficial ALT/WDL.