We reviewed the records of 209 patients with acute diverticulitis treated from 1984 through 1989 to determine if immunocompromised patients have a worse prognosis than nonimmunocompromised patients. Forty immunocompromised patients and 169 nonimmunocompromised patients with acute diverticulitis were identified. Free perforation into the peritoneal cavity occurred in 43% (17/40) of immunocompromised patients and 14% (24/169) of nonimmunocompromised patients. Operations were performed in 58% (23/40) of immunocompromised patients and 33% (55/169) of nonimmunocompromised patients. Postoperative morbidity was 65% (15/23) in immunocompromised patients and 24% (13/55) in nonimmunocompromised patients; postoperative mortality was 39% (9/23) and 2% (1/55), respectively. We conclude that acute diverticulitis in the immunocompromised patient is a complicated problem; there is a greater risk of free perforation and need for surgery than in the nonimmunocompromised patient. Furthermore, the prognosis for immunocompromised patients who undergo surgery is worse than that for nonimmunocompromised patients.