Background/Aims: Acute complicated diverticulitis can be subdivided into moderate diverticulitis and severe diverticu￢litis. Although there have been numerous studies on the risk factors for complicated diverticulitis, little research has focused on severe diverticulitis. This study was designed to identify the risk factors for severe diverticulitis in an acute diverticulitis attack using the modified Hinchey classification. Methods: Patients were included if they had any evidence of acute diverticulitis detected by computed tomography. The patients were subdivided into severe diverticulitis (Hinchey class ≥Ib; abscesses or peritonitis) and moderate diverticuli￢tis (Hinchey class Ia; pericolic inflammation) groups. Results: Of the 128 patients, 25 exhibited severe diverticulitis, and 103 exhibited moderate diverticulitis. In a multivariate analy￢sis, age >50 years (odds ratio [OR], 5.27; p=0.017), smok￢ing (OR, 3.61; p=0.044), comorbidity (OR, 4.98; p=0.045), leukocytosis (OR, 7.70; p=0.003), recurrence (OR, 4.95; p=0.032), and left-sided diverticulitis (OR, 6.92; p=0.006) were significantly associated with severe diverticulitis. Con￢clusions: This study suggests that the risk factors for severe diverticulitis are age >50 years, smoking, comorbidity, leuko￢cytosis, recurrent episodes, and left-sided diverticulitis.