ML-236B, a competitive inhibitor of 3-hydroxy-3-methylglutaryl CoA reductase, was administered to 11 patients with primary hypercholesterolemia. After 4--8 weeks of drug treatment at doses of 50--150 mg/day, serum cholesterol levels were reduced by 11--37% (27% on average) in cases of heterozygous familial hypercholesterolemia and combined hyperlipidemia. A marked reduction in tuberous xanthomas was noticed in a homozygous case of familial hypercholesterolemia, but here the drug was less effective in reducing the serum cholesterol level and a higher dose was required for treatment. Softening of Achilles tendon xantomas was observed in a case of combined hyperlipidemia.