Effects of long-term treatment with metoprolol and hydrochlorothiazide on plasma lipids and lipoproteins. In order to evaluate the effects of one-year antihypertensive treatment on plasma lipids and lipoproteins, 65 patients whose diastolic blood pressure was in the range 95-120 mmHg were randomly allocated to groups that received either hydrochlorothiazide or metoprolol, or both drugs when the response to one of them was insufficient to control blood pressure. Blood pressure was effectively reduced in all groups. Patients on hydrochlorothiazide showed a significant increase (P less than 0.01) in low-density lipoprotein cholesterol (LDL-C) after 3 months of treatment. A significant increase in triglycerides was observed after 6 and 12 months, together with a decrease in high-density lipoprotein cholesterol (HDL-C) after 12 months (P less than 0.05) of treatment in patients on metoprolol. In patients treated with both hydrochlorothiazide and metoprolol, total cholesterol increased after 3 (P less than 0.001) and 6 months (P less than 0.05), triglycerides increased after 6 (P less than 0.01) and 12 months (P less than 0.01), and LDL-C increased after 3 (P less than 0.05), 6 (P less than 0.001) and 12 months (P less than 0.01) of treatment, respectively. In 61% of the patients, three or more lipid parameters were affected during the study period. We conclude that long-term antihypertensive treatment with hydrochlorothiazide, metoprolol, and particularly with both drugs, can induce lipid effects that deserve recognition, because in some cases these might counteract the possible benefit of a reduction in blood pressure on the prevention of coronary heart disease. 