Recent improvements in instruments permit endoscopic examination of previously inaccessible sites. We report on the clinical use of a small-diameter endoscope to examine the spinal subarachnoid space, cord surface and syrinx cavities. Prior to clinical application, three types of endoscopes with external diameters of 0.5, 1.4 or 2.2 mm were inserted percutaneously in the lumbar region of five cadavers for preclinical evaluation of the procedure and the three endoscopes. The observations permitted us to perform spinal endoscopy preoperatively or intraoperatively using the 0.5-mm instrument in seven patients with spinal cord lesions between 1995 and 1997. The patients included two with spinal cord herniation through a dural defect, two with syringomyelia, one with spinal arachnoid cyst, one with spinal epidural cyst and one undergoing lumboperitoneal shunt for hydrocephalus. In patients in whom an endoscope was used preoperatively, the endoscope provided morphological information useful in preoperative diagnosis and planning surgical strategy. When the endoscope was used intraoperatively, areas outside the field of vision of a microscope could be examined, and physiological evaluation could include visualizing improved cord perfusion from the spinal subarachnoid space after surgery. Endoscopes could be safely inserted and approached to the lesions under direct vision while avoiding blood vessels and nerve roots on the spinal cord surface. No changes in symptoms or complications occurred in association with endoscopy. Using a small-diameter endoscope, the contents of the spinal subarachnoid space could be examined. Further improvements to increase possible endoscopic manipulation and enhance safety may extend the possibilities for endoscopic examination and permit endoscopic treatment.