Oxygen radicals play an important role in the course of experimental and human carcinogenesis. Antioxidant enzymes have been shown to protect cells against oxidative damage and, thus, to inhibit both initiation and promotion of carcinogenesis. The antioxidant enzyme superoxide dismutase catalyses the dismutation of superoxide anion, a very toxic radical, into hydrogen peroxide and exists as two types: one containing manganese and found in the mitochondria matrix and one containing copper-zinc and found in the cytosol. The superoxide dismutase activity has been shown to be impaired in some human neoplasm and in rat hepatocarcinoma. We found, in a recent study, a progressive reduction of superoxide dismutase activity progressing from chronic liver diseases to hepatocellular carcinoma; this finding seems consistent with current knowledge on the natural history of hepatocellular carcinoma.