This study describes the efficacy of the drug simvastatin. It is likely to be the first HMG CoA reductase inhibitor in Australia and New Zealand available for the treatment of hyperlipidemia. Twenty-four patients, 12 men and 12 women with primary hypercholesterolemia were randomly allocated to treatment by cholestyramine (eight patients) or to simvastatin (16 patients) for a 12-week period. With simvastatin, total cholesterol levels decreased by 37.5% from a baseline mean of 10.33 mmol/L to 6.4 mmol/L after 12 weeks. Low density lipoprotein (LDL) cholesterol concentration decreased by 48.2% from 8.40 mmol/L to 4.39 mmol/L. These effects were better than observed for cholestyramine alone where cholesterol and LDL-cholesterol reductions were 24.9% and 33.1% respectively. Thirteen patients, however, did not achieve target LDL levels of 3.62 mmol/L, or below, and therefore were treated with a combination of cholestyramine and simvastatin, resulting in a decrease of total cholesterol and LDL-cholesterol by 45.5% and 53.5% of baseline values studied over an eight-week period. No major clinical side-effects were encountered. One patient appeared to have had a change in colour vision at the end of the study at 20 weeks, without loss of visual acuity.