The development of spinal endoscope using a thin (0.75 mm in diameter) flexible fiber catheter AS-001, Fukuda-densi Co Ltd., Japan) is described in this study. This fiber catheter contains 3,000 optic fibers as imaging and illuminating fibers within its diameter. The effective length and the visual angle is 1.10 m and 53 degree, respectively. Video processor system was connected to this fiber catheter. With a patient in the lateral decubitus position under local anesthesia, this fiber catheter was introduced into the lumbar subarachnoid space in a similar manner as lumbar spinal drainage. Eight patients with myelopathy were evaluated by this spinal endoscopic study. With other radiological diagnostic modalities such as myelography, selective spinal angiography using intraarterial digital subtraction angiography (DSA), or magnetic resonance imaging (MRI), it was difficult to make definitive diagnosis in these 8 patients. In 4 patients among them, vascular tangle, or tortuous course of the dilated vessels were visualized on the dorsal surface of the spinal cord by the spinal endoscopic study, which strongly suggested the spinal arteriovenous malformation. In 3 of them, operative findings verified these preoperative endoscopic diagnoses to be correct. In the remaining case, surgical intervention was not attempted because of the personal affair of the patient. In every case, several structures around the thoracolumbar cord such as spinal dura mater, arachnoid membrane, or conus medullaris were observed under the pulsation of the cerebrospinal fluid. No complication was associated with this spinal endoscopic study. About 10 minutes were required for the whole procedure.(ABSTRACT TRUNCATED AT 250 WORDS)