The frequency of admission for acute terminal ileitis, Crohn's disease, and acute mesenteric lymphadenitis has been compared from the years 1951 to 1970, and the long-term results of operation for acute terminal ileitis in 1951-1965 have been examined. Whereas the admission rates for both acute terminal ileitis and Crohn's disease rose during the review period, that for acute lymphadenitis tended to fall slightly. Sixty-eight of 72 patients operated upon by appendicectomy who had acute terminal ileitis were followed up. One of these, with small intestinal stenosis at the initial operation, proved to have histological evidence of Crohn's disease, and a further nine have since developed unequivocal evidence of the disease. A cutaneous fistula developed in only one patient after appendicectomy. It is concluded that appendicectomy can safely be performed in acute terminal ileitis provided that the caecum is not inflamed and that Crohn's disease seldom develops as a sequel.