This prospective study examined factors which may predict a poor outcome (complications and recurrence) after a first attack of diverticulitis which has been successfully managed conservatively. Twenty-four of 107 patients who entered the study had a poor outcome: persistent diverticulitis (nine cases), recurrence (seven cases), colonic stenosis (six cases), residual parasigmoid abscess (one case) and colovesical fistula (one case). Eight of the 18 men aged 50 years or less had a poor outcome compared with 16 of the remaining 89 patients (P = 0.032). Twelve of 76 patients (16 per cent) with mild findings on computed tomography (CT) (localized thickening of colonic wall and inflammation of pericolic fat) had a poor outcome compared with 11 of 23 patients (48 per cent) whose CT was estimated as severe (abscess and/or extraluminal air and/or extraluminal Gastrografin) (P = 0.004). These results suggest that elective colectomy can be proposed after a first attack of acute left diverticulitis in men up to 50 years of age and/or in patients whose initial CT reveals findings of severe diverticulitis.