Forty-six cases of anomalous lumbosacral nerve roots were found in a series of 2123 patients who underwent myelography with water-soluble contrast medium. The anomalies were classified into five types. In Types I and II, one or more nerve roots emerged from the theca at a more cranial (Type I) or caudal (Type II) level than normal. In Type III, two or more roots emerged through closely adjacent openings in the dura, whereas in Type IV two nerve roots emerged from the dural sac combined as one nerve trunk. In Type V, two roots were connected by an anastomotic branch shortly after their emergence from the dura. The anomalies were usually unilateral and the fifth lumbar and first sacral-nerve roots were the most frequently involved. Type-III and Type-IV anomalies were the most common (69 per cent). Twenty-one per cent of the patients had lumbosacral anomalies and one had congenital absence of the articular facets of the lumbosacral joint on the side of the nerve-root anomaly. In seven patients the anomalous root or roots were compressed by a herniated disc, and the roots were entrapped in a lateral recess or intervertebral foramen in two. Symptoms and signs of nerve-root compression were usually severe, even in the patients with a small disc protrusion. The severity of the clinical findings appeared to be due to the reduced mobility of the anomalous roots. Adequate surgical decompression led to satisfactory final results in all nine patients.