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To understand why the population-based incidence of diverticulitis has increased over time, we studied temporal changes in age, body mass index (BMI), and diverticulitis in Olmsted County, Minnesota.We compared the BMIs of 2967 patients with diverticulitis and 9795 people without diverticulitis from January 1, 1980, through December 31, 2007. Because BMI is a surrogate for adipose tissue, computed tomographic estimations of abdominal fat content were compared between 381 diverticulitis cases and 381 age- and sex-matched controls.Between 1980 and 2007, the prevalence of obesity increased from 12% to 49% in the population and from 19% to 40% in patients with diverticulitis (P<.001 for both). Temporal trends in age, BMI, and the increased incidence of diverticulitis in people with normal BMI accounted for 48%, 47%, and 20%, respectively, of corresponding trends in diverticulitis. The secular decline in the proportion of people with normal BMI was partly offset by an increased incidence of diverticulitis in such people. In the case-control study, BMI was greater in cases than in controls (P=.001). However, after incorporating abdominal visceral (odds ratio [OR], 2.4; 95% CI, 1.6-3.7) and subcutaneous (OR, 2.9; 95% CI, 1.7-5.2) fat content (both associated with diverticulitis), BMI was associated with lower risk (OR, 0.8; 95% CI, 0.7-0.8) of diverticulitis.Aging, increasing obesity, and the increased incidence of diverticulitis in people with normal BMI account for the temporal increase in diverticulitis. Rather than BMI per se, increased abdominal visceral and subcutaneous fat are independently associated with diverticulitis. The incidence of diverticulitis, which is among the most common gastrointestinal diagnoses in hospitalized patients, has increased markedly since 2000. This study suggests that aging, increasing obesity, and the increased incidence of diverticulitis in people with normal BMI account for the temporal increase in diverticulitis.