Paget's disease frequently involves the spine. It rarely causes neurological complication resulting from compression of intraspinal nerve tissues. We describe a case of Paget's disease with spinal stenosis due to monostotic involvement of the third lumbar vertebra. The patient was troubled by low back pain and bilateral sciatica with progressive weakness of the left lower limb. Because of failure of calcitonin therapy and marked spinal stenosis at the 3rd lumbar vertebra level noted on roentgenographic examination, decompressive laminectomy was scheduled. Her symptoms were great relieved and her ambulating capability was similarly improved after posterior decompression.