The benefits of surgical correction of moderate internal carotid artery stenosis have been demonstrated only in symptomatic subjects. It is debatable whether patients with lacunar infarct ipsilateral to a moderate carotid stenosis may be considered symptomatic like those with large-artery stroke. The aim of the study was to seek markers capable of differentiating patients with lacunar or non-lacunar stroke ipsilateral to a moderate internal carotid artery stenosis. We enrolled 95 patients with a first stroke ipsilateral to a moderate (50-69 %) stenosis of the internal carotid artery and divided them into lacunar and non-lacunar stroke based on clinical presentation and neuroradiological findings; 34 subjects with asymptomatic moderate carotid stenosis and 31 normal individuals were also studied. Baseline characteristics; risk factors, cerebrovascular reactivity to hypercapnia evaluated by means of the breath-holding index (BHI), the presence and severity of carotid stenosis and intimamedia thickness (IMT) of the common carotid arteries were determined. There were 36 patients with lacunar and 59 with non-lacunar stroke. Degree of stenosis, and IMT and BHI ipsilateral to symptomatic stenosis were found to be significant independent predictors as each 10 % increase of stenosis carried a 4.3 higher probability of non-lacunar stroke (95 % CI: 1.91-9.51); each decimillimeter increment in IMT increased this probability by 1.45 (95 % CI: 1.10-1.92); and the risk odds ratio associated with each 0.1 increase in BHI was 1.88 (95 % CI: 1.33-2.66). A decrease in BHI of 0.1 thus carried a 90% greater probability of having a lacunar stroke. The results show that patients with moderate internal carotid artery stenosis and lacunar stroke can be differentiated from those with non-lacunar stroke on the basis of distinctive ultrasonographic findings. Further studies are needed to clarify whether our findings have pathogenetic implications and may be of help for the planning of different therapeutic strategies in patients with moderate internal carotid stenosis and lacunar or non-lacunar ipsilateral stroke.