Introduction: While hepatocellular carcinoma (HCC) can arise spontaneously, the majority of cases are observed in patients with cirrhosis or chronic hepatitis infection. Hepatocellular adenoma is also an important cause of liver mass, but is often found in women and associated with exposure to estrogens. Adenomas can be seen in males, especially those with history of anabolic androgen use. Focal nodular hyperplasia (FNH) is another benign cause of liver mass usually seen in females. We report a case of an undifferentiated liver mass in a healthy male patient that was originally thought to be FNH but turned out to be a more serious condition. Case Description/Methods: An asymptomatic 36-year-old man with unremarkable medical history presented with a liver mass after routine blood work revealed an elevated alkaline phosphatase. Risk factors for liver disease included obesity and hyperlipidemia. Imaging demonstrated a mass in the right hepatic lobe (8.1 x 7.1 x 9.0 cm), with arterial hyperenhancement, followed by isointensity to the liver after the arterial phase. The surrounding parenchyma appeared normal. He had no exposure to hepatitis B or C and his chronic liver disease evaluation was negative. Alpha fetoprotein was normal. Given these findings, the lesion was initially deemed most consistent with FNH. Multidisciplinary review instead favored adenoma or HCC, due to male gender, lesion size, T2 hyperintensity relative to liver, and central areas of high signal intensity that did not fill like a classic central scar. The patient subsequently underwent right hepatectomy. Surgical pathology revealed a well-differentiated hepatocellular neoplasm without invasion of the surrounding vasculature. The lesion had a prominent capsule and consisted of sheets and nodules of well-differentiated hepatic cells. The remainder of the liver parenchyma showed mild steatosis without inflammation or fibrosis. Pathology was reviewed at multiple institutions, with results favoring a well-differentiated HCC or atypical adenoma. Unfortunately, it was difficult to determine whether this mass was benign or malignant. Discussion: In this case, presentation at a multidisciplinary conference was lifesaving. Given that adenomas of this size carry high risk for malignant transformation or rupture, and large HCC often metastasize and can be terminal, surgery was indicated regardless of diagnosis. However, FNH can typically be observed. The patient now follows with Oncology and Hepatology for surveillance imaging and labs every 3 months.Figure 1.: A. Contrast-enhanced MRI demonstrates a large (8.1 x 7.1 x 9.0 cm) right hepatic lobe mass with heterogeneous hyperenhancement in the arterial phase. B. Gross pathology of 9.7cm tumor in right hepatic lobe. C. High magnification of the tumor (H&E) showing unpaired arteries without abnormal thickening of hepatocellular plates.