To investigate two rare benign lesions, hepatocellular adenoma (HCA) and focal nodular hyperplasia (FNH), and evaluate differential diagnosis Methods Twenty-four consecutive patients with presumed HCA and FNH were studied at the Liver Cancer Institute from January 1996 to May 1999 Preoperative assessment included clinical evaluation, symptoms and laboratory tests New imaging techniques were prospectively appraised in addition to usual techniques All had hepatic resections and follow-up Histologic examination of surgical specimens was obtained in all cases Results In every instance, FNH was an incidental finding FNH consists of nodular aggregates of cytologically normal hepatocytes with foci of intranodular bile duct proliferation In this series, patients with HCA had larger tumors and more often were symptomatic but the occurrence was unrelated to oral contraceptive steroids (OCS) usage Intralesional hemorrhage or necrosis is common, and was seen in 75% of cases The best imaging procedure in the diagnosis of FNH was MRI Color Doppler US was a useful adjunct, but CT lacked specificity, making histological diagnosis mandatory All patients underwent tumor resected were tumor——free during the follow-up Conclusions FNH is a distinct histopathologic entity, and is distinguishable from HCA FNH is a hyperplastic response by the liver parenchyma to a pre-existing arterial malformation HCA is a liver neoplasia and has the potential of malignant transformation to HCC Based on these findings, we believe that if the clinical suspicion of HCA or FNH is strong, resection is usually the best approach if technically feasible and histologic diagnosis is mandatory To investigate two rare benign lesions, hepatocellular adenoma (HCA) and focal nodular hyperplasia (FNH), and evaluate differential diagnosis Methods Twenty-four consecutive patients with presumed HCA and FNH were studied at the Liver Cancer Institute from January 1996 to May 1999 Preoperative assessment included clinical evaluation, symptoms and laboratory tests New imaging techniques were prospectively appraised in addition to usual techniques All had hepatic resections and follow-up Histologic examination of surgical specimens was obtained in all cases Results In every instance, FNH was an incidental finding FNH consists of nodular aggregates of cytologically normal hepatocytes with foci of intranodular bile duct proliferation In this series, patients with HCA had larger tumors and more often were symptomatic but the occurrence was unrelated to oral contraceptive steroids (OCS) usage Intralesional hemorrhage or necrosis is common, and was seen in 75% of cases Th e best imaging procedure in the diagnosis of FNH was MRI Color Doppler US was a useful adjunct, but CT lacked specificity, making histological diagnosis mandatory all All patients underwent tumor resected were tumor - free during the follow-up Conclusions FNH is a distinct histopathologic entity , and is distinguishable from HCA FNH is a hyperplastic response by the liver parenchyma to a pre-existing arterial malformation HCA is a liver neoplasia and has the potential of malignant transformation to HCC Based on these findings, we believe that if the clinical suspicion of HCA or FNH is strong, resection is usually usually the best approach if technically feasible and histologic diagnosis is mandatory