In the present study, we examined the levels of plasma lipids and apolipoproteins in patients with non-insulin dependent diabetes mellitus (NIDDM) with hypercholesterolemia in different apolipoprotein E (apo E) phenotypes. We also examined the influences of apo E polymorphism on the response to pravastatin. The patients were divided into three groups, E4/E3, E3/E3, and E3/E2. There were no differences in the baseline levels of plasma lipids and apolipoproteins, except that the level of triglycerides in E3/E2 heterozygotes was significantly higher than E3/E3 homozygotes. Three months of pravastatin administration significantly reduced plasma levels of total cholesterol and low-density lipoprotein cholesterol in each group to the same degree. We observed a significant reduction of apo B both in the E4/E3 and E3/E3 groups and apo E in the E3/E3 group. Such reduction was not observed in the E3/E2 group. We conclude that pravastatin is a potent drug to correct lipid abnormalities, particularly in NIDDM patients with apo E4/E3 and E3/E3. In the E3/E2 group, its effectiveness may be diminished.