We studied 85 cancer patients with lumbosacral plexopathy and documented pelvic tumor by CT or biopsy. Three clinical syndromes were delineated: lower (L4-S1), 51%; upper (L1-L4), 31%; and pan-plexopathy (L1-S3), 18%. Seventy percent of patients had the insidious onset of pelvic or radicular leg pain, followed weeks to months later by sensory symptoms and weakness. The quintet of leg pain, weakness, edema, rectal mass, and hydronephrosis suggests plexopathy due to cancer. CT showed pelvic tumor in 96%. On myelography, epidural extension, usually below the conus medullaris, was seen in 45%. With treatment, only 28% of patients had objective responses on CT and 17% on examination.