We studied 20 patients with lumbosacral radiculoplexopathy from radiation treatment and 30 patients with plexus damage from pelvic malignancy. Indolent leg weakness occurred early in radiation disease, whereas pain marked the onset of tumor plexopathy. Eventually, all radiation cases had weakness, which was bilateral in most of them and painless in one-half of them. Tumor patients typically had unilateral weakness, which was painful in all of them. Radiation disease often resulted in serious neurologic disability. Of the tumor patients, 86% were dead within 31/2 years after onset of neurologic symptoms.