Six cases of cyst of the ligamentum flavum with compression of a lumbar nerve root are reported. All patients exhibited recurrent back pain and sciatica. Investigation included computed tomography, myelography, or both. The correct diagnosis was reached before operation in only half the patients. High-resolution computed tomography performed in the four last patients outlined the cystic lesion with its low-density center. Surgical excision was performed in all patients. Microscopic examination showed a dense fibrous cyst arising from the ligamentum flavum. The lumen contained myxoid or necrotic material, but no epithelial lining. Cysts of the ligamentum flavum must be considered in the differential diagnosis of causes of sciatica. A firm radiological diagnosis may, at present, still require myelography combined with high-resolution computed tomography. Differentiation from synovial or ganglion cysts of the spine is discussed.