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In this paper, we investigate the possibility of using the fractal dimension to characterise carotid atheromatous plaques from B-mode ultrasound images. The images were obtained from ten symptomatic and nine asymptomatic subjects. Symptomatic subjects included patients with previous history of cerebral events, whereas asymptomatic ones had no evidence of any cerebral symptoms prior to the time of the investigation. For each subject, a sequence of images was collected corresponding to 2-3 cardiac cycles. The boundary of the atheromatous plaque was defined by an expert in three different images of each sequence, corresponding to systole, diastole and a random phase of the cardiac cycle. The fractal dimension of each plaque was estimated using a new method, namely the k-th nearest neighbour method. The results showed that the values of the fractal dimension in the symptomatic group were significantly higher than those in the asymptomatic group. These results suggested that the fractal dimension, estimated from B-mode ultrasound images, could be used to discriminate between symptomatic and asymptomatic carotid atheromatous plaques.