Aim <p>To identify factors predicting the failure of non-operative treatment in acute complicated colonic diverticulitis.</p> Material and methods <p>Consecutive patients hospitalized for non-operative treatment of acute complicated diverticulitis of the sigmoid colon between 2009 and 2015 were included in this retrospective analysis. Complicated disease was defined as the presence of extraluminal air or fluid collection within a computed tomography (CT) scan. The primary endpoint of the study was the need for emergent sigmoidectomy. The direction of perforation was assessed by CT scan and divided into 2 main groups: perforation towards the small bowel and perforation in other directions (abdominal wall, pelvic wall, retroperitoneum, urogenital organs).</p> Results <p>A total of 140 patients were included. Of these patients, 25 patients did not respond to non-operative treatment and underwent rescue surgery (18%). CT revealed perforations towards the small bowel in 28 patients, 19 of whom did not respond to non-operative treatment (68%); in contrast, 6 of 112 (5%) patients with perforation in other directions experienced treatment failure. By multivariate analysis, perforation towards the small bowel (hazard ratio 75.0; 95% CI, 13.7–409.7, <i>p</i> &lt; 0.001) was associated with a significantly increased risk for a failure of non-operative management. The only other risk factor was the presence of an intra-abdominal abscess. Diverticular perforation towards the small bowel is associated with a very high risk for emergency sigmoidectomy due to failed non-operative treatment.</p> Conclusion <p>Diverticular perforation towards the small bowel is associated with a very high risk for emergency sigmoidectomy due to failed non-operative treatment.</p>