All cases of primary appendiceal mucoceles (simple mucocele, cystadenoma, cystadenocarcinoma) diagnosed between 1976 and 2000 were reviewed. There were 135 patients, 74 of whom were female. Mean age at diagnosis was 59 years. Mean follow-up was more than 6 years.Interventions: A total of 129 patients underwent surgery, consisting of appendectomy (22 patients), right hemicolectomy (25 patients), or more extensive procedures (82 patients).Main Outcome Measures: Clinical, diagnostic, and surgical variables were statistically compared with post-operative morbidity and mortality and the presence of malignancy. PϽ.05 was considered significant.Results: The presence of symptoms was associated with malignancy (58% vs 15%, PϽ.001), particularly abdominal pain (56% vs 29%, P=.005) and weight loss (77% vs 31%, P=.002). Abdominal mass was also associated with malignancy (86% vs 25%, PϽ.001). Moreover, pseudomyxoma peritonei and mucocele extravasation were associated with malignancy (95% vs 13%, PϽ.001, and 83% vs 15%, PϽ.001, respectively). The lesion size was not associated with malignancy; however, cystadenomas were significantly larger than simple mucoceles (8.1 cm vs 4.1 cm, PϽ.001), and no cystadenoma was less than 2 cm in largest diameter.Conclusions: A number of clinical, diagnostic, and intraoperative findings are associated with malignant mucoceles. All mucoceles greater than 2 cm should be excised to remove premalignant lesions.