Nineteen patients with plexus lesions following radiation therapy were investigated: fifteen with brachial plexus, 4 with lumbar or sacral plexus involvement. Symptoms at onset are usually sensory. Motor disturbances occur either simultaneously or after some delay, their course is generally gradual and unfavourable. Areflexia appears early and was present in every case. Important cutaneous lesions (radiodermitis) and considerable induration of soft tissues were observed in every patient. Diagnosis is a relapse of the mitotic process. Severity of prognosis makes imperative a definite technique of radiation therapy. In all the patients included in this study, dosage had exceded 1,600 rets. Patients were tentatively treated with D-penicillamine, an inhibitor of collagen synthesis.