Patients were studied after a first episode of acute left-colonic diverticulitis for the initial and later evolution of the disease with the aim of defining evidence-based indications for elective surgery. Relevant data from prospective studies were retrieved from a MEDLINE search of English language articles. Young male patients (≤ 50 years of age) had a higher risk of CT-graded severe diverticulitis. After medical treatment of the first episode, the incidence of complications was highest for young patients with CT-graded severe diverticulitis and lowest for older patients with CT-graded moderate diverticulitis. Recurrence in the form of diffuse peritonitis was rare. CT grading of initial diverticulitis seemed to be a predictor of recurrence, whereas the role of age was less clear. A family history of diverticulitis might be predictive of recurrence. CT grading of acute diverticulitis helps to predict poor outcome after medical treatment of a first episode. Elective surgical resection should be proposed to patients with residual symptoms who do not respond to conservative treatment. Additional research is needed to clarify the role of a genetic predisposition in the development of diverticulitis in young adults.