The distribution of carotid plaque ultrasound appearance has been evaluated using duplex ultrasound in symptomatic and asymptomatic patients. There were 134 patients with unilateral carotid territory symptoms who subsequently underwent endarterectomy of the symptomatic carotid bifurcation, and 92 asymptomatic patients. Both carotid bifurcations in all patients were examined, thus providing three groups of vessels for study: (i) asymptomatic vessels in asymptomatic patients (n = 184); (ii) asymptomatic vessels in symptomatic patients (n = 134); and (iii) asymptomatic contralateral vessels in symptomatic patients (n = 134). Ultrasound appearances were classified as types 1-4. This classification has previously been compared prospectively with endarterectomy specimen pathology where the more echolucent type 1 and 2 lesions correlated well with the presence of intraplaque haemorrhage or ulceration. In the symptomatic arteries, type 1 and 2 lesions were predominant, whereas in the asymptomatic patients the most common lesions were types 3 and 4. This difference was statistically significant (P less than 0.01). Evaluation of the asymptomatic contralateral vessel in the symptomatic patients showed a pattern of plaque type distribution between the other two groups.