This work presents a computerized lesion segmentation technique on breast ultrasound images. There were applied known techniques such as morphological filtering, Watershed transform and average radial derivative function. To evaluate the performance of the proposed method the resulting segmentation contours were compared with 36 breast US images manually delineated by two senior radiologists. Further, two evaluation parameters were used: the percentage of coincidence (CP) and the proportional distance (PD). The former indicates the similarity between contours, while the latter express the dissimilarity. The accuracy of the proposed method was evaluated by considering images with CP ges 80% and PD les 10% as adequately delineated.