PURPOSE: To evaluate the frequency and the cause of pathological false negative results on ultrasound-guided biopsy of hepatic masses. MATERlALS AND METHODS: We performed US-guided needle biopsy of hepatic masses in 245 patients from January 1992 to June 1996. Twenty-four among 245 patients had false negative results pathologically. We retrospectively analyzed the ultrasound scans of 24 patients on the basis of size, site, and echo pattern of the hepatic masses. The final diagnoses were hepatocellular carcinoma (n=11), cholangiocarcinoma (n=8), other malignant tumor (n=2) (metastatic adenocarcinoma 1, undifferentiated carcinoma 1), and benign lesion (n=3) (adenomatous hyperplasia 1, hemangioma 1 and hepatic abscess 1). Final diagnosis was based on rebiopsy in 8, surgery in 5, and radiological and clinical diagnosis in 11 cases. RESULTS: The false negative rate was 9.8 % (24/245) and those of hepatocellular carcinoma, cholangiocarcinoma, other malignant tumor, and benign lesion were 8.5 % (11/129), 19.0 % (8/42), 5.3 % (2/38), and 8.3 % (3/36), respectively. The size of masses were 1-5 cm (mean 2.9 cm) in hepatocellular carcinoma, 4-6 cm (mean 5.3 cm) in cholangiocarcinoma, 3-8cm (mean 5.0cm) in other malignant tumor, and 2-4 cm (mean 3.0 cm) in benign lesion. Six of hepatocellular carcinomas were located in liver dome. Three of cholangiocarcinomas were located in the atrophic left lobe and were difficult to distinguish from surrounding normal liver parenchyma. The echo pattern of the masses was hypoechoic 6, hyperechoic 4 and isoechoic 1 in hepatocellular carcinoma, and hypoechoic 5 and hyperechoic 3 in cholangiocarcinoma. CONCLUSION: False negative rate of fine needle aspiration biopsy of liver mass was 9.8% and was the highest in cholangiocarcinoma. The incidence of false negative result was high when the hepatocellular carcinoma was smaller than 3cm and located in liver dome and when the cholangiocarcinoma was located in atrophic liver and had necrosis and fibrosis.