There is no established treatment for steatohepatitis in patients who are not alcoholics. This disease is a potentially progressive liver disease associated with hepatic insulin resistance. Only a weight-reducing diet in overweight patients has proved effective. We treated 20 patients who had steatohepatitis but were not alcoholics with metformin (500 mg three times a day for 4 months), an agent that improves hepatic insulin sensitivity. When compared with the six individuals not complying with treatment, long-term metformin significantly reduced mean transaminase concentrations, which returned to normal in 50% of actively-treated patients. Also, insulin sensitivity improved significantly and liver volume decreased by 20%. Similar data have been reported in insulin-resistant ob/ob mice with fatty liver. A randomised-controlled study is needed.