Recent advances in imaging modalities enable the identification of small hepatocellular nodules. Among the imaging techniques currently used for detecting hepatocellular carcinomas (HCC), computed tomography (CT) during arterial portography (CTAP) is one of the most sensitive techniques available for detecting hemodynamic change. Even so, well-differentiated HCCs that display only limited hemodynamic change, a feature shared with nonmalignant hepatocellular nodules, are not always detectable by CTAP. To improve our ability to distinguish well-differentiated HCCs from nonmalignant hepatocellular nodules, we have attempted to clarify how the characteristics of the nodules are shown by each imaging technique. We studied the imaging and pathological characteristics of 31 nodules (in 22 patients) detected by ultrasonography (US), but not by CTAP. Histological diagnoses were as follows: HCC, 17 of 31 nodules (55%); high-grade dysplastic nodules, 1 of 31 (3%); and nonmalignant nodules, 13 of 31 (42%). Neither digital substraction angiography (DSA) nor CT arteriography (CTA) were able to detect any of the nodules. Detection rates for plain CT were: 5 of 17 (29%) HCC, 1 of 1 (100%) high-grade dysplastic nodules, and 1 of 13 (8%) nonmalignant nodules. Detection rates for T1/T2-weighted magnetic resonance imaging (MRI) were: 4 of 17 (24%) HCC, 1 of 1 (100%) high-grade dysplastic nodules, and 3 of 13 (23%) nonmalignant nodules. Dynamic CT and dynamic MRI provided no additional information. In conclusion, there is some probability that hepatocellular nodules detected by US, but not by CTAP, are HCC. Presently, it is difficult to distinguish between benign nodules and malignant ones with these imaging techniques, and our findings indicate that biopsy may be advisable for nodules detected under these conditions.