OBJECTIVE: A systematic review of the nonsurgical treatment of patients with appendiceal abscess or phlegmon, with emphasis on the success rate, need for drainage of abscesses, risk of undetected serious disease, and need for interval appendectomy to prevent recurrence. SUMMARY BACKGROUND DATA: Patients with appendiceal abscess or phlegmon are traditionally managed by nonsurgical treatment and interval appendectomy. This practice is controversial with proponents of immediate surgery and others questioning the need for interval appendectomy. METHODS: A Medline search identified 61 studies published between January 1964 and December 2005 reporting on the results of nonsurgical treatment of appendiceal abscess or phlegmon. The results were pooled taking the potential clustering on the study-level into account. A meta-analysis of the morbidity after immediate surgery compared with that after nonsurgical treatment was performed. RESULTS: Appendiceal abscess or phlegmon is found in 3.8% (95% confidence interval (CI), 2.6-4.9) of patients with appendicitis. Nonsurgical treatment fails in 7.2% (CI: 4.0-10.5). The need for drainage of an abscess is 19.7% (CI: 11.0-28.3). Immediate surgery is associated with a higher morbidity compared with nonsurgical treatment (odds ratio, 3.3, CI: 1.9-5.6, P < 0.001). After successful nonsurgical treatment, a malignant disease is detected in 1.2% (CI: 0.6-1.7) and an important benign disease in 0.7% (CI: 0.2-11.9) during follow-up. The risk of recurrence is 7.4% (CI: 3.7-11.1). CONCLUSIONS: The results of this review of mainly retrospective studies support the practice of nonsurgical treatment without interval appendectomy in patients with appendiceal abscess or phlegmon. © 2007 Lippincott Williams & Wilkins, Inc.