Since 1954, 34 patients have attended St. Mark's Hospital with pyoderma gangrenosum in association either with ulcerative colitis (22 patients) or Crohn's disease (12 patients). Lesions were multiple in 71 per cent and over half were situated below the knees. Ulcerative colitis was active in 11 patients (50 per cent) and Crohn's disease was active in nine (75 per cent) when pyoderma gangrenosum was diagnosed. Associated illnesses--most commonly a seronegative arthritis affecting large joints--were present in 55 per cent and 92 per cent of cases respectively. A diffuse pustular rash appeared in six patients, synchronously with pyoderma in five. In a further seven patients (two with ulcerative colitis, five with Crohn's disease) the onset or course of pyoderma might have been linked to the presence of non-dermatological suppuration. Pyoderma resolved without intestinal resection in two-thirds of patients. When present at the time of surgical resection (15 procedures in 13 patients), pyoderma healed promptly in six cases, only with additional therapy in four cases and very slowly or not at all in five cases. Pyoderma gangrenosum occurs in both ulcerative colitis and Crohn's disease. Healing after intestinal resection is unpredictable both with respect to timing and extent of resection.