Here we report 42 cases of postirradiation lesions of the brachial nerve plexus in patients treated with radiotherapy after radical mastectomy. These lesions usually appeared at least 1 year after treatment, and motorial disturbances always developed, even if initially they may have been absent. A comparison of the case material indicates that a reduction in the dose administered to the supraclavicular region and exclusion of the axillary region from the treatment resulted in a significant reduction in the incidence of these lesions. In fact, in a group of 490 patients treated by the old radiotherapy method at the Istituto Nazionale Tumori of Milan (i.e., administration of 6000 rad to the brachial nerve plexus), 16 cases of lesions at this plexus were observed. In a group of 200 patients treated instead with a new method (i.e., administration of 4900 rad to the brachial nerve plexus and exclusion of the axillary region), no nerve lesions were observed, with significance levels, according to Fisher's test, less than 0.9%. Since radiolesions at the brachial nerve plexus have a very bad prognosis with regard to functioning of the limb and physiotherapy is of little help, this new therapeutic method is very useful, and it is not accompanied by an increase in the number of local recurrences as compared to the old method.