In a prospective evaluation of 140 consecutive patients with acute left-sided colonic diverticulitis demonstrated by computerized tomography (CT) in all cases, 22 (16 percent) were found to have an associated abscess without peritonitis. Thirteen of these 22 required surgery (seven during the first stay and six from 2 to 11 months after the acute episode; median, three months). Nine patients were treated conservatively, eight of whom are now totally asymptomatic 24 months after the initial attack (range, 10-47 months). There were 10 mesocolic abscesses (seven treated with antibiotics alone), nine pelvic abscesses (seven requiring surgery), and three intra-abdominal abscesses, all operated upon. These results suggest that mesocolic abscesses can usually be managed conservatively without drainage; should surgery be necessary, en bloc resection with immediate anastomosis can usually be safely performed. Pelvic and intraabdominal abscesses behave more aggressively and usually require a two-stage surgical procedure when initial percutaneous drainage cannot be performed or is felt to be hazardous.