Background: The effect of body mass index (BMI) and smoking on the risk of perforated appendix and postoperative complications in patients undergoing open appendicectomy for acute appendicitis was studied. Results: After adjustment for age, calendar period and BMI, smoking was significantly associated with an increased risk of perforated appendicitis (PA) (P = 0路004). The relative risk was 1路29 (95 per cent confidence interval 1路11 to 1路50) among current smokers with more than 10 pack-years of tobacco use. In patients with non-perforated appendicitis (NPA), the relative risk of overall postoperative complications was significantly associated with BMI (P < 0路001), and was 2路60 (1路71 to 3路95) in obese patients and 1路51 (1路03 to 2路22) in current smokers with more than 10 pack-years of tobacco use. In patients with PA, overweight, obesity and smoking status were not associated with an increased risk of overall postoperative complications.Conclusion: Perforation due to acute appendicitis was associated with current tobacco smoking. A BMI of 27路5 kg/m 2 or more and current smoking were associated with overall postoperative complications in patients with NPA.