Preoperative distinction between focal nodular hyperplasia (FNH) that should be managed conservatively and hepatocellular adenoma (HA) that should be resected remains difficult. The result is controversial management of these patients. The aims of this study were to report the value of modern imaging procedures for noninvasive diagnosis of these lesions, to assess the value of intraoperative frozen section studies, and to propose a management strategy in those patients. Fortyone consecutive women with FNH (35 cases) or HA (6 cases) treated at our institution between 1985 and 1992 were studied. New imaging techniques, including enhanced magnetic resonance imaging (MRI) and color Doppler ultrasonography (US), were prospectively appraised in addition to usual techniques. Histological examination of surgical specimens was obtained in all cases. A sixfold increase in the number of patients with FNH was observed during this study, whereas the number of patients with HA did not change. FNHs were incidental US findings in 74% of the cases. The best imaging procedure in the diagnosis of FNH was enhanced MRI with a sensitivity of 70% and a specificity of 98%. Color Doppler US was a useful adjunct. Intraoperative frozen section studies were performed in 16 patients with 19 tumors with a sensitivity of 89% and a specificity of 100%. From this study, we have come to the following conclusions: (1) in women, FNH is now much more often detected than HA; (2) using enhanced MRI, a preoperative diagnosis of FNH is possible in 70% of the cases, avoiding unnecessary surgery; (3) when clinical, biochemical, or imaging data are not typical of FNH, histological diagnosis is mandatory and can be safely and consistently made on large surgical biopsy specimens; (4) during surgery, a decision to resect or not resect a lesion can be efficiently assisted by frozen-section studies. (Hepatology 1995; 22:1674-1681).