OBJECTIVE: To evaluate the effect of obesity on the incidence of adverse events (surgical site infection, dislocation, re-revision, or > or =1 adverse event), functional outcome, residual pain, and patient satisfaction after revision total hip arthroplasty (THA). METHODS: We conducted a university hospital-based prospective cohort study including 52 obese and 152 nonobese patients with revision THA performed between 1996 and 2006. We used incidence rates, rate ratios, and hazard ratios (HRs) to compare the incidence of events in obese and nonobese patients and in 4 body mass index (BMI) categories (<25, 25-29.9, 30-34.9, > or =35). Functional outcome and pain were measured 5 years postoperative using the Harris Hip Score. RESULTS: The incidence rate for > or =1 complication increased with rising BMI (1.8, 3.4, 10.3, and 17.9 cases/100 person-years). The increase was small between normal and overweight patients (adjusted HR 1.5, 95% confidence interval [95% CI] 0.5, 4.7), significantly greater with BMI 30-34.9 (adjusted HR 4.5, 95% CI 1.4, 14.0), and most evident with BMI > or =35 (adjusted HR 10.9, 95% CI 2.9, 41.1). The adjusted HR for surgical site infection (obese versus nonobese) was 4.1 (95% CI 1.1, 15.0) and for dislocation 3.5 (95% CI 1.3, 9.3). Eighty patients had a followup visit at 5 years. Obese patients had moderately lower functional results and higher levels of residual pain, but patient satisfaction was almost similar. CONCLUSION: Revision THA is technically challenging, particularly in obese patients, probably due to more difficult anatomic conditions. We found an increased risk of adverse events, notably surgical site infection and dislocation in these patients.