The mechanisms underlying iron-induced liver fibrogenesis in patients with genetic hemochromatosis are poorly understood. We studied signs of Kupffer cell activation and inflammatory responses in liver biopsy specimens obtained from 15 patients with untreated and six patients with treated hemochromatosis. Immunohisto-chemistry was performed on 11 of the untreated and all treated patients. Three of the untreated patients (20%) had cirrhosis and eight (53%) had fibrosis. None had chronic active hepatitis (CAH). Immunohistochemistry indicated that 55% of the untreated patients had sparse intercellular adhesion molecule-1 (ICAM-1) expression by hepatocytes, and all of these had Kupffer cell iron overload. No ICAM-1 expression was seen by hepatocytes in treated patients or healthy controls. ICAM-1 was strongly expressed by hepatocytes from control patients with inflammatory liver disease. HLA-DR reactivity was seen on sinusoidal cells in all groups, but not on hepatocytes except for two of the control patients with CAH. Twenty-seven percent of the untreated hemochromatosis patients displayed moderate infiltration by CD3-positive lymphocytes. Electron microscopy of samples from untreated hemochromatosis patients showed hypertrophic Kupffer cells containing iron-rich remnants of phagocytosed hepatocytes. Fat-storing cells close to iron-laden hepatocytes contained multiple lipid droplets and adjacent collagen fibril bundles. Thus, in patients with untreated genetic hemochromatosis and Kupffer cell iron overload, hepatocytes occasionally express ICAM-1. In regions with heavy iron overload, Kupffer cell hypertrophy and transition of fat-storing cells are seen. Our findings indicate that release of factors from iron-loaded, activated Kupffer cells is of importance for the transformation of fat-storing cells and increased collagen deposition seen in genetic hemochromatosis.