Systemic vasculitis is known to affect the gastrointestinal tract but the nature of the complication is poorly characterized. Out of 65 patients with systemic vasculitis, the majority of whom had renal disease, the intestine was found to be affected in 18. These comprised four of eight patients with polyarteritis nodosa, nine of seventeen with microscopic polyarteritis, four of thirty-six with Wegener's granulomatosis and one of four with Churg-Strauss syndrome. The features included abdominal pain (85 per cent), diarrhoea (50 per cent), gut haemorrhage (44 per cent) and abnormal liver function tests (50 per cent). Manifestations of gastrointestinal disease were evident at presentation in half the patients and led to a fetal outcome in five. Ileus, mucosal abnormalities, perforation and slow transit were evident radiographically, and selective visceral angiography showed aneurysms or organ infarcts in five patients. Histological assessment of gut biopsies (chiefly rectal) revealed non-specific inflammation or ulceration in nine patients and intramucosal haemorrhage in two. Focal areas of necrosis and ulceration in colonoscopic biopsies were highly suggestive of vasculitis whereas arteritis was only found in one full thickness biopsy. Hence the diagnosis of gastrointestinal complications depends largely on clinical evidence. In patients who survived, the gastrointestinal features remitted as the systemic illness improved following treatment with steroids, cyclophosphamide or plasma exchange.