Between January 1987 and May 1988 a prospective study was carried out on 232 women with a palpable breast mass. They underwent physical examination, x-ray mammography and ultrasound mammography. The results of each study were interpreted independently by separate observers, and consensus was achieved. From all solid tumours histology was available. Compared to x-ray mammography, ultrasound mammography proved to be significantly (p less than 0.025) more sensitive in diagnosing malignant palpable breast masses (98.7% versus 86.8%). However there was no statistical significant difference in specificity between ultrasound mammography and x-ray mammography. Using ultrasound mammography we classified 9 histological proven malignant tumours as such which were negative on plain x-ray mammography. Despite the good results of ultrasound mammography in diagnosing palpable breast lesions it can never replace x-ray mammography in cases of malignancy, because the results of ultrasound mammography with non-palpable synchronous tumours with or without microcalcifications are poor. In some cases of benign lesions diagnosed with ultrasound mammography no x-ray mammography is necessary.