Hepatocellular carcinoma (HCC) is frequently associated with paraneoplastic hypercholesterolemia. However, cholesterol overproduction in HCC tissue has never been demonstrated. An aim of this study is to prove cholesterol overproduction in the HCC tissue of patients with paraneoplastic hypercholesterolemia. Six patients with HCC associated with paraneoplastic hypercholesterolemia and three control patients with HCC who did not have hypercholesterolemia were investigated regarding the expression of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase in HCC tissue by means of immunohistochemical technique. In HCC associated with paraneoplastic hypercholesterolemia, HMG-CoA reductase was clearly stained in cancer cells whereas surrounding non-tumorous hepatocytes showed only slight expression of HMG-CoA reductase. In contrast, HCC tissues derived from patients without paraneoplastic hypercholesterolemia showed only slight expression of HMG-CoA reductase whereas surrounding non-tumorous hepatocytes showed a clear expression of HMG-CoA reductase. We morphologically proved cholesterol overproduction in HCC tissue derived from patients with paraneoplastic hypercholesterolemia. Immunohistochemistry for HMG-CoA reductase thought to be useful in the diagnosis of paraneoplastic hypercholesterolemia.