Mucoceles of the appendix and associated pseudomyxoma peritonei are a heterogeneous group comprising various histopathologic lesions with differing prognoses. Between 1983 and 1990, we treated eight patients with appendiceal mucocele, three cystadenomas and five cystadenocarcinomas, three of which had accompanying pseudomyxoma peritonei. All patients were more than 50 years of age. Women outnumbered men by seven to one. Preoperative diagnosis was acute appendicitis or appendiceal abscess in all instances of mucocele unaccompanied by pseudomyxoma peritonei. Ultrasound of the abdomen, together with paracentesis, diagnosed pseudomyxoma peritonei in two of three patients. Elevated carcinoembryonic antigen levels were found in six of the eight patients and monitoring of this parameter was useful in the early detection of the two recurrences observed. The three patients with cystadenomas remain free of disease after appendectomy. Of the five patients treated for cystadenocarcinoma by right colectomy, two underwent reoperation after recurrence of disease. One patient died 41 months later of intestinal obstruction caused by pseudomyxoma peritonei. Pseudomyxoma peritonei significantly decreases survival of patients with appendiceal mucocele. In these patients, aggressive initial surgical management, repeated if need be, is indicated.