The first clinical studies on hydroxy-methyl-glutaryl-coenzyme A (HMG-CoA) reductase inhibitors reported a low incidence of liver toxicity. The personal observation of a case of simvastatin-induced acute cholestatic hepatitis prompted us to evaluate the true incidence of hepatic side effects of these drugs in a large Italian population. One hundred patients taking simvastatin and ninety patients treated with pravastatin were followed-up six months with periodical evaluation of serum lipid levels and liver function test. In 5% of simvastatin-treated patients and 4.5% of pravastatin-treated patients significant liver toxicity was observed, which required drug discontinuation. The mechanism of liver damage was direct, idiosyncratic, but immunological features of drug toxicity could be hypothesized in one patient.