Plasma lipid, lipoprotein, and apoprotein concentrations were measured in 169 patients with non-insulin-dependent diabetes mellitus (NIDDM), 78 with normal blood pressure, and 91 diagnosed and receiving drug treatment for hypertension. Plasma triglyceride, cholesterol, low-density lipoprotein-cholesterol, and apoprotein B concentrations were significantly higher (p = less than 0.05 to less than 0.001) in the hypertensive group. In addition, the ratios of high-density to low-density lipoprotein-cholesterol and of apoprotein A-1 to apoprotein B were significantly reduced (p less than 0.01) in patients with hypertension. The changes noted were independent of differences in sex distribution, degree of obesity, and level of glycemic control. These results indicate that substantial differences in plasma lipid, lipoprotein, and apoprotein concentrations are seen when normotensive patients with NIDDM are compared with patients who are also being treated for hypertension, and that all of the changes noted would increase the risk of coronary artery disease in the hypertensive group. Since all patients with hypertension were receiving anti-hypertensive medications, it is not clear if it is hypertension per se, or its treatment, that is responsible for the observed changes in lipid metabolism.