Details of 300 patients with complicated diverticular disease from 30 hospitals between 1985 and 1988 were entered into a national audit organized by the Surgical Research Society. Complications present on admission included acute phlegmon (n = 104), pericolic abscess (n = 34), purulent peritonitis (n = 40), large bowel obstruction (n = 31), faecal peritonitis (n = 23), pericolic abscess complicated by fistula (n = 28) and lower gastrointestinal bleeding (n = 40). The overall mortality rate was 11.3 per cent (acute phlegmon, 4 per cent; purulent peritonitis, 27 per cent; pericolic abscess, 12 per cent; faecal peritonitis, 48 per cent; large bowel obstruction, 6 per cent; bleeding, 2 per cent; fistula, 4 per cent). Acute phlegmon was treated without operation in 78 patients (75.0 per cent) and by resection in 24 (23.1 per cent). Management of purulent peritonitis generally involved Hartmann's procedure (62 per cent) or resection and primary anastomosis (15 per cent). Similarly, patients with pericolic abscess usually underwent Hartmann's procedure (38 per cent) or resection and primary anastomosis (35 per cent). The principal operation for faecal peritonitis was Hartmann's resection (83 per cent). Large bowel obstruction was managed conservatively in four patients (13 per cent), by Hartmann's procedure in nine (29 per cent), and by resection and primary anastomosis with or without a proximal stoma in 13 (42 per cent). Most patients (82 per cent) with fistula associated with an abscess were managed by resection and primary anastomosis; 90 per cent with acute gastrointestinal bleeding were treated without operation.