CS-514, one of the derivatives of ML-236B which is an inhibitor of endogenous cholesterol synthesis, has been previously shown to effectively reduce low density lipoprotein (LDL) cholesterol in dogs, rabbits and humans. We determined the effect of CS-514 on glucose, lipid, lipoprotein and apolipoprotein (apo) levels in plasma of 8 hypercholesterolemic diabetics (2 males). Total and LDL cholesterol and apo B levels were significantly decreased (P less than 0.005) 3 months after CS-514 treatment. High density lipoprotein (HDL) cholesterol was increased (P less than 0.05). Fasting blood glucose (FBG) and hemoglobin A1c (HbA1c) did not change throughout the observation period. No clinically serious adverse effects were experienced by the patients. We conclude that CS-514 can be a useful drug in the treatment of hypercholesterolemic diabetics and is remarkably free of any evidence of toxicity or unwanted side effects even in diabetics.