Accurate localization of a tumour is imperative before excision biopsy for impalpable breast lesions seen on screening mammography. Ultrasound localization has the advantages of being simple, quick and noninvasive. In a 3-year period from September 1989, 185 women have required localization of a breast tumour prior to biopsy, 159 were performed using a wire localization technique and 26 (14%) were performed using ultrasound alone. Twenty-two out of the 26 were malignant lesions. The mean maximum diameter of tumour diagnosed by ultrasound compared to histological sample was 10.6 mm (8.5-12.7*) to 11.1 mm (8.9-13.3*) with a correlation of r = 0.87. In the malignant cases the mean maximum diameter of tissue excised was 63.0 mm (57.2-65.9*). Complete excision was obtained in 21 out of the 22 patients with malignant disease by one operation. Ultrasound alone has been used successfully as a means of preoperative localization in selected cases prior to excision biopsy of a breast tumour.