The occurrence of the C3F allele was investigated in the following three groups: 69 consecutive referred patients with untreated essential hypertension, including borderline hypertension; 70 patients with established and treated essential hypertension, already attending the same outpatient clinic, and 62 age- and sex-matched normotensive healthy subjects without clinical signs of atherosclerosis or familial predisposition to hypertension. In the three groups the C3F allele was found in 38.2%, 29% and 20%, respectively. Among the treated hypertensive patients with C3F gene, 40% had coronary heart disease (CHD) compared to 6.1% among the C3F negative (P less than 0.005), and the relative risk of CHD among the treated hypertensive patients with this allele was found to be 10.2 (P less than 0.002). The C3F gene was present in 72.7% of the treated patients with CHD. In the untreated patients the occurrence of CHD was low, and no differences between C3F positive and negative patients could be demonstrated. No association of the C3F allele with familial predisposition to hypertension was found. This study provides further evidence of a positive association of the C3F allele with atherosclerosis, and it is concluded that this allele in a hypertensive patient might accelerate the atherosclerotic process, with subsequent premature development of vascular complications.