Background: Perforated diverticulitis with purulent peritonitis has traditionally been treated with open colon resection and stoma formation with risk for reoperations, morbidity, and mortality. Laparoscopic lavage alone has been suggested as definitive treatment. Objective: To compare laparoscopic lavage with open colon resection and colostomy (Hartmann procedure) for perforated diverticulitis with purulent peritonitis. Design: Randomized, controlled, multicenter, open-label trial. (ISRCTN registry number: ISRCTN82208287) Setting: 9 hospitals in Sweden and Denmark. Patients: Patients who have confirmed Hinchey grade III perforated diverticulitis with purulent peritonitis at diagnostic laparoscopy. Intervention: Randomization between laparoscopic lavage and the Hartmann procedure. Measurements: Primary outcome was the percentage of patients having 1 or more reoperations within 12 months. Key secondary outcomes were number of reoperations, hospital readmissions, total length of hospital stay during 12 months, and adverse events. Results: A total of 43 and 40 patients were randomly assigned to laparoscopic lavage and the Hartmann procedure with a median (first, third quartiles) follow-up of 372 days (336, 394) and 378 days (226, 396), respectively. Fewer patients in the laparoscopic group (12 of 43; 27.9%) than in the Hartmann group (25 of 40; 62.5%) had at least 1 reoperation within 12 months (relative risk reduction, 59%; relative risk, 0.41 [95% CI, 0.23 to 0.72]; P = 0.004). Mortality and severe adverse events did not differ between groups. Total length of hospital stay (days) within 12 months was shorter for the laparoscopic group than the Hartmann group, with a reduction of 35% (relative risk, 0.65 [CI, 0.45 to 0.94]; P = 0.047). After 12 months, 3 patients in the laparoscopic group and 11 in the Hartmann group had a stoma. Limitation: Not all patients presenting with suspected diverticulitis were enrolled. Conclusion: Laparoscopic lavage reduced the need for reoperations, had a similar safety profile to the Hartmann procedure, and may be an appropriate treatment of choice for acute perforated diverticulitis with purulent peritonitis. Primary Funding Source: ALF; Sahlgrenska University Hospital, Gothenburg.