To determine whether the presence of an extruded lumbar disc prolapse could be predicted from clinical symptoms, the authors compared the relative proportions of back and leg pain, with operative findings in a prospective observational study of 100 lumbar discectomy patients. All cases were assessed by an independent observer, blind to the knowledge of the operative findings. Of the 58 men and 42 women (mean age 42, range 19–75), 47 patients had a subligamentous disc protrusion and 53 had an extruded disc fragment. Of 27 patients who presented with leg pain only, 26 (96%) were found subsequently to have an extruded fragment. Patients with an extruded fragment had a significantly higher proportion of leg: back pain (median 99: 1) than those with a subligamentous disc protrusion (median 75: 25, P < 0.001). Patients with leg pain only and those with a marked predominance of leg pain over back pain have a high probability of harboring an extruded disc fragment.