Five hundred and sixty-four patients reviewed 1 year after major abdominal surgery have been studied prospectively by a single observer for 10 years to determine the incidence and significance of incisional hernia. Of 337 (60 per cent) patients completing the 10 year follow-up 37 (11 per cent) developed an incisional hernia and 13 (35 per cent) of these first appeared at 5 years or later. One in three hernias caused symptoms. The late appearing hernias were smaller than the early ones, and caused little trouble. Of the 18 patients who consulted their general practitioner, 11 had symptoms and of these six (55 per cent) were referred for surgical opinion. Many hernias were diagnosed at routine outpatient follow-up and were likely to receive treatment from the surgeon. Most symptomatic patients were offered surgery with the remainder usually being offered a corset. In about half our patients (mainly those without symptoms) surgery was refused or advised against although the patients would have accepted it. Recurrence is common after surgical repair (40 per cent) but seems to be related to surgical technique. The possibility of complications occurring from an incisional hernia does not appear to be discussed with patients although obstruction occurred in 14 per cent of our patients with troublesome hernia.