The primary objective in this multicentre, double-blind randomised, parallel study was to compare the metabolic effects of 12 months of treatment with an ACE inhibitor (enalapril) with those of a selective beta-blocker (atenolol) in patients with mild hypertension. The patients (35-69 years of age) were included if they were without antihypertensive drugs and after six months of nonpharmacological treatment had supine DBPs between 90 and 104 mmHg; 220 patients were randomised to enalapril (20 or 40 mg/day) and 218 to atenolol (50 or 100 mg/day). After 12 months of treatment, atenolol significantly increased the glucose concentrations at 60, 90 and 120 minutes after an oral intake of 75 g glucose (P < 0.01), while enalapril did not. Atenolol significantly increased fasting blood glucose and insulin concentration 120 minutes after glucose intake, while enalapril did not. Plasma total cholesterol and triglycerides increased significantly in patients having atenolol but not in those having enalapril. HDL cholesterol decreased significantly in the atenolol group but not in the enalapril group. The proportions of patients with clinical adverse experiences were similar in both treatment groups. These results indicate that enalapril does not influence either glucose tolerance or lipoprotein metabolism while atenolol does. These findings are consistent over the 12 month treatment period and confirm earlier short-term study results.