The authors discuss the clinical and therapeutic problems of post irradiation lesions of the brachial plexus resulting from radiotherapy in the treatment of breast cancer. Twelve such cases were referred to our clinic in the 2 year period ending June 1980. The results after 2 years are reported and the literature on the subject is reviewed. Surgical treatment aimed at mobilising the nerve roots (neurolysis) is recommended, but we would stress that the best treatment is prophylactic. With more strict control of the radiotherapeutic technique, particularly with regard to the total dosage administered, it should never happen