Investigation of the sagittal diameter of the spinal canal by ultrasound has shown a relative stenosis in patients with sciatica. In this study 12 patients who had recovered uneventfully after being operated on for herniated lumbar discs were found to have a broader spinal canal than 10 patients treated in the same way but still suffering from pain. Moreover, more difficulty was experienced investigating this latter group with the described "B-scan" technique. It is suggested that a preoperative ultrasound investigation would be able to indicate a narrow spinal canal and thereby reduce the number of disc operation failures.