BACKGROUND: Operative approaches for Hinchey III diverticulitis include the Hartmann procedure, primary resection and anastomosis, and laparoscopic lavage. Several randomized controlled trials and meta-analyses have compared these approaches; however, results are conflicting and previous studies have not captured the complexity of balancing surgical risks and quality of life. OBJECTIVE: This study aimed to determine the optimal operative strategy for patients with Hinchey III sigmoid diverticulitis. DESIGN: We developed a Markov cohort model, incorporating perioperative morbidity/mortality, emergency and elective reoperations, and quality-of-life weights. We derived model parameters from systematic reviews and meta-analyses, where possible. We performed a second-order Monte Carlo probabilistic sensitivity analysis to account for joint uncertainty in model parameters. SETTING: This study measured outcomes over patients' lifetime horizon. PATIENTS: The base case was a simulated cohort of 65-year-old patients with Hinchey III diverticulitis. A scenario simulating a cohort of highly comorbid 80-year-old patients was also planned. INTERVENTIONS: Hartmann procedure, primary resection and anastomosis (with or without diverting ileostomy), and laparoscopic lavage were performed. MAIN OUTCOME MEASURES: Quality-adjusted life years were the primary outcome measured. RESULTS: Following surgery for Hinchey III diverticulitis, 39.5% of patients who underwent the Hartmann procedure, 14.3% of patients who underwent laparoscopic lavage, and 16.7% of patients who underwent primary resection and anastomosis had a stoma at 12 months. After applying quality-of-life weights, primary resection and anastomosis was the optimal operative strategy, yielding 18.0 quality-adjusted life years; laparoscopic lavage and the Hartmann procedure yielded 9.6 and 13.7 fewer quality-adjusted life months. A scenario analysis for elderly, highly comorbid patients could not be performed because of a lack of high-quality evidence to inform model parameters. LIMITATIONS: This model required assumptions about the long-term postoperative course of patients who underwent laparoscopic lavage because few long-term data for this group have been published. CONCLUSIONS: Although the Hartmann procedure is widely used for Hinchey III diverticulitis, when considering both surgical risks and quality of life, both laparoscopic lavage and primary resection and anastomosis provide greater quality-adjusted life years for patients with Hinchey III diverticulitis, and primary resection and anastomosis appears to be the optimal approach.