The authors report their experience from 1985 to 1988 with 75 consecutive patients affected by bilateral carotid artery stenosis in whom only one side was surgically treated and the other had a minor (15-45%) asymptomatic carotid stenosis. These patients have been followed for a period ranging from 10 to 50 months (mean follow-up 21 months) by clinical examinations and non-invasive investigations (Doppler CW, Duplex scanner). The non-invasive evaluation included assessment of haemodynamic data and characterisation of plaque morphology (regular vs. irregular or ulcerated surface, homogeneous vs. heterogeneous plaque). During follow-up eight patients died: two (2.6%) from acute myocardial infarction, four from stroke (5.3%), and two (2.6%) from other causes. Twenty-five patients (33.3%) had neurological symptoms related to the unoperated side: and four suffered stroke (5.3%). Twenty-one patients had TIAs (28%) related to the observed side. During follow-up five out of 29 (17.2%) homogeneous and 20 out of 46 (43.4%) heterogeneous plaques progressed (p less than 0.01). With regard to the surface characteristics, nine out of 25 regular plaques progressed and only three patients (12%) had neurological events; 19 out of 34 (55.9%) irregular plaques showed a progression and 14 caused neurological symptoms. Fourteen ulcerated plaques (87.5%) progressed (p less than 0.01). Our experience suggests that the basic trend in the follow-up of patients with bilateral carotid artery disease, is that the contralateral unoperated lesion may evolve and become symptomatic. These symptoms are generally TIAs but five patients (4.3%) suffered from strokes related to the unoperated side, all without warning TIAs.