BACKGROUND: Focal nodular hyperplasia (FNH) is increasingly diagnosed as a result of the advances in imaging studies such as contrast-enhanced ultrasonography (CEUS), enhanced computed tomography and magnetic resonance imaging. However, FNH with atypical features can be difficult to differentiate from other benign and malignant tumors. The aim of this study was to investigate the influence of fatty liver background on the CEUS characteristics of FNH. METHODS: Twenty-six patients with FNH were divided into two groups: group A included 14 patients with fatty liver and group B included 12 patients with normal liver background. Conventional two-dimensional ultrasonography and color Doppler flow imaging (CDFI) were conducted and followed by real-time dual-frame CEUS. RESULTS: On two-dimensional ultrasonography, hypo-echoic nodules were present in most of the patients in group A (12/14) and hyperechoic nodules in most of those in group B (7/12). The difference in the nodule echotextures between the two groups was statistically significant (P<0.05). Nodules with centrifugal blood flow signals on CDFI were found in 6 of the 14 patients in group A and 5 of the 12 in group B (P>0.05). On CEUS, nodules with a central spoked-wheel-like enhancement pattern in the early arterial phase were observed in 8 patients in group A and those with an eccentric enhancement pattern in the remaining 6 patients. In this group, 3 patients had hypoechoic nodules in the delayed phase. Eleven of the 14 patients in this group were diagnosed accurately with CEUS. In group B, nodules with a rapid central spoked-wheel-like enhancement pattern in the early arterial phase were found in 8 patients by CEUS and those with rapid an eccentric enhancement pattern in 4. The nodules were found to be continuously enhanced in the delayed phase. All of the patients in group B were accurately diagnosed with CEUS. CONCLUSIONS: A FNH nodule on a background of fatty liver may present a hypoechoic pattern on two-dimensional ultrasonography and a hypoechoic wash-out pattern in the delayed phase on CEUS. At this time, punch biopsy is needed for the diagnosis or differential diagnosis of FNH. BACKGROUND: FNH with isypically diagnosed as a result of the advances in imaging studies such as contrast-enhanced ultrasonography (CEUS), enhanced computed tomography and magnetic resonance imaging. However, FNH with atypical features can be difficult to differentiate from the aim of this study was to investigate the influence of fatty liver background on the CEUS characteristics of FNH. METHODS: Twenty-six patients with FNH were divided into two groups: group A included 14 patients with fatty liver and group B included 12 patients with normal liver background. Conventional two-dimensional ultrasonography and color Doppler flow imaging (CDFI) were conducted and followed by real-time dual-frame CEUS. RESULTS: On two-dimensional ultrasonography, hypo-echoic nodules were present in most of the patients in group A (12/14) and hyperechoic nodules in most of those in group B (7/12). The difference in the nodule echotextures betw Nodules with centrifugal blood flow signals on CDFI were found in 6 of the 14 patients in group A and 5 of the 12 in group B (P> 0.05). On CEUS, nodules with a central spoked-wheel-like enhancement pattern in the early arterial phase were observed in 8 patients in group A and those with an eccentric enhancement pattern in the remaining 6 patients. In this group, 3 patients had hypoechoic nodules in the delayed phase. Eleven of the 14 patients in this group were diagnosed accurately with CEUS. In group B, nodules with a rapid central spoked-wheel-like enhancement pattern in the early arterial phase were found in 8 patients by CEUS and those with rapid an eccentric enhancement pattern in 4. The nodules were found to be continuously enhanced in the delayed phase. All of the patients in group B were accurately diagnosed with CEUS. CONCLUSIONS: A FNH nodule on a background of fatty liver may present a hypoechoic pattern on two-dimensional ultrasonography and a hypoechoic wash-out pattern in the delayed phase on CEUS. At this time, punch biopsy is needed for the diagnosis or differential diagnosis of FNH.