The known shift in collagen synthesis from procollagen type I to type III in patients with atherosclerosis, suggested measurement of serum procollagen III peptide (PIIIP) levels in patients with coronary artery disease (CAD). Two groups of patients were studied: group I--thirty-six patients with CAD (male, mean age 56.9 +/- 7.5 years, hospitalized for coronary angiography. Risk factors included 16 patients with high blood pressure, four diabetics, 31 smokers and 15 with hypercholesterolaemia. Five patients had no significant lesions, seven had one vessel with over 50% stenosis, 10 had two vessels and 14 had three vessels. Group II--35 patients (male, mean age 39.4 +/- 13.3 years), with normal physical examination and ECG according to WHO criteria, formed the control group: the risk factors included nine patients with high blood pressure, 14 smokers and one with hypercholesterolaemia. Procollagen III peptide levels were determined by radioimmunoassay. In group I, PIIIP levels were 26.8 +/- 16 ng ml-1 vs. 10.4 +/- 3.2 for group II. Sixty-one per cent of group I had pathological levels of PIIIP with an absence of correlation with the severity of atherosclerosis or risk factors. Only 2.8% of patients in group II had pathological levels. Procollagen III peptide determination would appear to be a sensitive, specific and predictive test for atherosclerosis in patients with CAD.