A retrospective review of 1,220 cystic fibrosis (CF) patients between 1965 and 1989 identified 60 patients who underwent appendectomy. Ten had appendectomy prior to referral and 16 had an incidental appendectomy (primarily meconium ileus). Among the remaining 34 patients, acute appendicitis was present in 19 (1.8% incidence). "Classic appendicitis"--acute abdominal pain shifting to the right lower quadrant (RLQ), focal RLQ tenderness, and elevated white blood cell (WBC) count--was present in 15. Four additional inflamed appendixes were removed in patients following incorrect preoperative diagnosis. Thirteen of these 19 were perforated. Complications included wound infection (2), pelvic abscess (1), ileal obstruction requiring ileostomy (1), and pelvic hematoma (1). There were no deaths. One patient with acute RLQ pain and tenderness had a normal appendix. Seven patients (mean, 20 years) had chronic, intermittent, focal RLQ pain and tenderness not originating periumbilically with a normal WBC count and temperature. At exploration, these appendixes were enlarged and tensely distended with inspissated mucus. Microscopic examination showed no inflammation. Appendectomy resulted in resolution of symptoms without complications. Four additional patients with intermittent RLQ pain and tenderness and a history of recurrent intussusception presented with ileocolic intussusception. Persistent postreduction symptoms in three and failure of reduction in the fourth necessitated celiotomy at which time tensely distended appendixes were removed. Appendectomy led to resolution of symptoms. Three additional enlarged noninflammed appendixes were incidentally removed in asymptomatic patients undergoing unrelated intraabdominal procedures. Appendiceal disease in CF patients represents a spectrum ranging from simple mucous distention to acute appendicitis with perforation. CF patients with pain secondary to a noninflamed distended appendix represent a distinct syndrome cured by appendectomy.(ABSTRACT TRUNCATED AT 250 WORDS)