Three patients are described who developed a brachial plexus neuropathy following radiation treatment for cancer of the breast. The clinical features consisted of a painless, slowly progressive sensory motor disturbance, affecting especially the hand. The latent period between the radiation therapy and the onset of the neuropathy was exceptionally long, being 8, 15 and 15 years respectively. Two patients were initially incorrectly diagnosed as having a carpal tunnel syndrome. The possible mechanisms of the insidious neuropathy are discussed.