Patients with carotid atherosclerosis are at increased risk of both stroke and ischaemic heart disease. Low-density lipoprotein (LDL) is a heterogeneous group of particles, with small, dense particles being more atherogenic. We studied 79 patients (51 men, mean +/- SD age 62.4 +/- 11.7 years) referred for Doppler ultrasound assessment of the carotid arteries. Evidence of carotid atherosclerosis, defined as the presence of atherosclerotic plaque, stenosis or occlusion in one or more of the six carotid artery segments examined, was found in 44 patients (56%). LDL subfractions were analysed by disc polyacrylamide gel electrophoresis with prior ultracentrifugation of serum to remove chylomicrons. This method produces a LDL score; the higher the score, the greater the proportion of the more atherogenic LDL subfractions. Mean LDL score was significantly higher in diseased patients (mean +/- SD, 1.56 +/- 0.61) than the normal group (1.26 +/- 0.65) (t = 2.12, p = 0.037). There was no significant association between LDL score and severity of carotid artery stenosis. Age (adjusted odds ratio 1.09, 95% CI 1.03-1.15) and smoking history (2.09, 95% CI 1.10-3.98) predicted carotid atherosclerosis in logistic regression analysis, with LDL score achieving borderline significance (2.20, 95% CI 0.91-5.29). Small, dense LDL subfractions are associated with carotid atherosclerosis and may be a modifiable risk factor for stroke as well as ischaemic heart disease.