Between 1977 and 1989, 151 patients were treated in our institution for acute sigmoid diverticulitis. Thirty-one patients were operated on for diffuse peritonitis, and were excluded from the study. One hundred twenty patients had localized disease. There were 59 men and 61 women, with a mean age of 60 years (range, 30 to 87 years). Thirteen were under 40 years of age. A "phlegmonous" diverticulitis (no pericolic abscess) was diagnosed in 78 cases (group I). A pericolic abscess was identified in 42 cases (group II). The medical treatment was successful in 97% of the patients of the group I. Only 15 patients required a delayed elective resection for recurrence or chronic complications, within the next 24 months. There were no operative deaths. All the other patients were doing well after a mean follow-up of 5 years (9-144 months), without any disease-related death. Patients presenting with a localized pericolic abscess (group II, n = 42) were initially treated either conservatively (n = 22) or by a more or less extensive drainage (n = 20). There were two deaths in the "conservative" group. Primary or delayed colonic resection was indicated in 34 cases because of uncontrolled sepsis, recurrence or secondary chronic complications. It is concluded that accurate classification of the disease is essential. If no peritonitis has developed, the presence of an abscess is the main determinant in both prognosis and treatment. Most patients who develop an acute phlegmonous diverticulitis do well with conservative treatment, and prophylactic resection is not indicated. Curative colectomy is reserved for patients developing persistent complications over the next few months.(ABSTRACT TRUNCATED AT 250 WORDS)