A pathomorphological study was conducted to clarify the localization of Kupffer cells in hepatocellular carcinoma (HCC) tissues and such hyperplastic nodular lesions as adenomatous hyperplasia (AH) and focal nodular hyperplasia (FNH). Materials were surgical specimens of 50 HCCs, 7 AHs, and 13 FNHs. These tissues were immunohistochemically stained with an anti-human macrophage antibody (anti-CD68 antibody). Among CD68-positive cells, short spindle-shaped cells were considered as Kupffer cells, and Kupffer cell numbers in tumor lesions and surrounding liver tissues of each specimen were comparatively examined. As a result, the number of Kupffer cells in well-differentiated HCC tissues less than 1 cm in diameter was 27.8 +/- 3.3 (mean +/- SE/025 mm2); in noncancerous tissues, it was 302 +/- 3.2, showing no statistically significant differences. The number of Kupffer cells in cancerous tissues decreased in comparison with the number in noncancerous tissues, as the tumor size increased and histological grade decreased. In hyperplastic nodular lesions, the number was higher in nodular lesions than in the surrounding liver tissues in 4 of 7 AHs (57.1%) and 6 of 13 FNHs (46.2%). This could explain the reason why enhanced MRI, which utilizes the selective taken-up mechanism of chondroitin sulfate iron colloid and superparamagnetic iron oxide into the reticuloendothelial system of the liver and spleen, depicts well-differentiated HCC and AH at the same signal intensity as in the surrounding liver tissues. Our findings also indicate that there is a limitation in differentiating or diagnosing small HCC and hyperplastic nodular lesions by using enhanced MRI, which utilizes Kupffer cell functions.