Summary Background Researchers have suggested that antibiotics could cure acute appendicitis. We assessed the efficacy of amoxicillin plus clavulanic acid by comparison with emergency appendicectomy for treatment of patients with uncomplicated acute appendicitis. Methods In this open-label, non-inferiority, randomised trial, adult patients (aged 18–68 years) with uncomplicated acute appendicitis, as assessed by CT scan, were enrolled at six university hospitals in France. A computer-generated randomisation sequence was used to allocate patients randomly in a 1:1 ratio to receive amoxicillin plus clavulanic acid (3 g per day) for 8–15 days or emergency appendicectomy. The primary endpoint was occurrence of postintervention peritonitis within 30 days of treatment initiation. Non-inferiority was shown if the upper limit of the two-sided 95% CI for the difference in rates was lower than 10 percentage points. Both intention-to-treat and per-protocol analyses were done. This trial is registered with ClinicalTrials.gov , number NCT00135603. Findings Of 243 patients randomised, 123 were allocated to the antibiotic group and 120 to the appendicectomy group. Four were excluded from analysis because of early dropout before receiving the intervention, leaving 239 (antibiotic group, 120; appendicectomy group, 119) patients for intention-to-treat analysis. 30-day postintervention peritonitis was significantly more frequent in the antibiotic group (8%, n=9) than in the appendicectomy group (2%, n=2; treatment difference 5·8; 95% CI 0·3–12·1). In the appendicectomy group, despite CT-scan assessment, 21 (18%) of 119 patients were unexpectedly identified at surgery to have complicated appendicitis with peritonitis. In the antibiotic group, 14 (12% 7·1–18·6) of 120 underwent an appendicectomy during the first 30 days and 30 (29% 21·4–38·9) of 102 underwent appendicectomy between 1 month and 1 year, 26 of whom had acute appendicitis (recurrence rate 26%; 18·0–34·7). Interpretation Amoxicillin plus clavulanic acid was not non-inferior to emergency appendicectomy for treatment of acute appendicitis. Identification of predictive markers on CT scans might enable improved targeting of antibiotic treatment. Funding French Ministry of Health, Programme Hospitalier de Recherche Clinique 2002.