OBJECTIVE Coronary arteries affected by atherosclerosis undergo focal compensatory enlargement, which can be detected by intracoronary ultrasound but not by angiography. Diabetic patients when compared with non-diabetics have a more accelerated progression of coronary artery disease and a more diffuse narrowing of the coronary arteries. Intracoronary ultrasound can clarify if this is due to less compensatory coronary artery enlargement as a response to atherosclerosis. RESULTS Ten non-diabetic and 15 diabetic patients with coronary artery disease, with angiographically determined one- or two-vessel disease, underwent intracoronary ultrasound examination of the non-stenotic coronary artery. Forty-five sites with luminal stenosis, detected by intracoronary ultrasound, were analysed (15 in non-diabetics, 30 in diabetics). Vessel and lumen area, atherosclerotic plaque area and plaque composition were evaluated. Vessel area was also measured proximal and distal to the healthy segment. In the diabetic patients, there was less vessel area increase from the proximal healthy segment into the atherosclerotic segment than in the non-diabetic patients (99% separate-variance confidence intervals for differences between diabetics' and non-diabetics' means = 0.29 mm2, 2.71 mm2). The proximal plaque free vessel area, the atherosclerotic plaque area and plaque composition were similar between the two groups. CONCLUSIONS Diabetics with atherosclerosis have less compensatory coronary artery enlargement than non-diabetics. This may explain the diffuse and accelerated course of coronary artery disease in these patients.