We report a 47-year-old woman who developed a slowly progressive lumbosacral plexopathy with mixed sensorimotor losses in the lower extremities. The symptoms were apparent 8 years after x-ray irradiation for an ovarian carcinoma. Neurological examination showed mild weakness and absent deep tendon reflexes of bilateral lower extremities, and hypesthesia to all modalities in anterior aspects of bilateral lower thighs, in dorsum pedis and soles. Extensive investigations regarding the possibility of tumor recurrence were negative. Computed tomography of pelvis showed abnormal soft tissue densities around the lumbosacral plexus. Intravenous pyelography showed bilateral hydronephrosis and narrowed ureters at the first sacral vertebra level. These findings are consistent with radiation-induced fibrosis rather than tumor infiltration. The results suggest the entrapment lumbosacral plexopathy due to surrounding fibrosis after irradiation. We speculated the sensorimotor losses caused by entrapment of the lumbosacral plexus.