An open, randomized, parallel clinical study comparing the antihypertensive efficacy and tolerability of the angiotensin converting enzyme inhibitor enalapril and the diuretic hydrochlorothiazide was carried out in 40 patients with mild uncomplicated essential hypertension. Changes in plasma lipid profiles were also studied. The results show that the two treatments were equally efficacious in controlling blood pressure and were generally well tolerated. Favorable changes in the plasma lipid profile were observed during the 12-week treatment period with enalapril. However, adverse alterations in the plasma lipid profile were observed during treatment with hydrochlorothiazide. These included a significant increase in the total cholesterol/high density lipoprotein cholesterol ratio and the low density lipoprotein cholesterol/high density lipoprotein cholesterol ratio. Further studies are needed to confirm these plasma lipid findings and to determine their clinical significance.