Four patients who had low-back pain and sciatica were diagnosed as having a lumbar intraspinal extradural synovial cyst adjacent to a facet joint between the fourth and fifth lumbar vertebrae. The patients ranged in age from forty-nine to seventy-one years, and the symptoms and signs involved the fourth or fifth lumbar-nerve roots. Roentgenographically, degeneration of the intervertebral discs and facet joints was noted in every patient. Degenerative spondylolisthesis was also a frequent finding. Myelography and computed tomographic scans aided in diagnosis, revealing a soft-tissue lesion, occasionally rimmed with calcification, adjacent to the involved facet joint. The treatment was surgical excision of the cyst, as well as complete laminectomy if there was concomitant spinal stenosis. Follow-up, ranging from eighteen to twenty-five months, revealed complete resolution of the sciatica in all patients.