Different doses of 3-hydroxy-3-methyl-glutaric acid (HMG) on plasma lipids were studied in a double-blind trial in 36 patients with familial hypercholesterolemia (type IIa or hyper-beta-lipoproteinemia). The patients were randomly assigned to five groups, and each group received one of the following treatments: placebo, or HMG 750 mg, 1500 mg, 2250 mg, or 3000 mg per day. As compared to placebo, the mean plasma cholesterol levels during the eight-week treatment period were 11 and 13 per cent lower in the 2250-mg and 3000-mg HMG-treated groups (P less than 0.034 and less than 0.021, respectively). At the same dosage levels LDL cholesterol was decreased by 8 per cent. HMG had no significant effect on plasma triglycerides as compared to placebo. Discontinuation of the medication did not result in a rebound of plasma cholesterol. There were no clinical or biologic adverse effects due to the administration of HMG, and all patients maintained excellent compliance to the medication. Because of its lack of toxicity, HMG may be useful as an adjunct to diet in the treatment of familial hypercholesterolemia.