Background The optimal treatment remains controversial for acute left-sided colon perforation. Therefore, the effectiveness and safety of primary anastomosis versus Hartmann's operation (HP) was compared in a case-matched control study. Methods Thirty consecutive patients with primary anastomosis and protective ileostomy (PAS) were matched to 30 HP patients, controlling for age, gender, American Society of Anesthesiologists (ASA) score, body mass index (BMI), and peritonitis severity (Hinchey). In a second analysis, PAS patients with purulent peritonitis (Hinchey 3) were matched to patients with primary anastomosis without ileostomy (PA). Results Hospital mortality was similar between HP (17%) and PAS (10%). Complication frequency and severity (requiring re-intervention or admission to the Intensive Care Unit [ICU]) were comparable for the first operation (60% versus 56% and 30% versus 32%). The stoma reversal rate was higher in PAS than in HP (96% versus 60%, p = 0.001), with significantly fewer complications (23% versus 66%, p = 0.02), and lower severity (7% versus 33%, p = 0.02). Additional analysis of PAS versus PA showed similar morbidity (52% versus 41%, p = 0.45) and complication severity (18% versus 24%, p = 0.51), whereas overall operation time and hospital stay were significantly shorter in PA (169 versus 320 min, p = 0.003, 17 versus 28 days, p < 0.001). Conclusions Primary anastomosis and protective ileostomy is a superior treatment to HP in acute left-sided colon perforation. In the absence of feculent peritonitis an ileostomy appears unnecessary.How to manage acute left-sided colon perforation remains controversial, as no study providing a high level of evidence is currently available. Although the superiority of an approach with primary resection has been established in multicenter randomized studies over the traditional threestage treatment (diversion, resection, and reanastomosis) [1], the indication to perform a primary anastomosis versus an end-colostomy (Hartmann's procedure; HP) is still under debate.Acute colonic perforation, mostly caused by diverticulitis [2][3][4], is associated with high rates of mortality (up to 20%) and morbidity (up to 60%) [3,5]. The prevalence of diverticular disease is increasing in Western countries, as approximately one-third of the population is affected by the sixth decade of life, and half by the ninth decade [6,7]. Diverticulitis occurs in 10%-25% of patients with diverticulosis [8], and