The clinical outcome and optimum classification of patients who have sigmoid resection specimens that show the histologic features of Crohn's disease (CD) and diverticulitis is not well defined. Historically, these patients were considered to have coexistent diseases, but recent studies have suggested that the CD-like changes are part of the inflammatory reaction of the diverticulitis. Sorting out these issues has been complicated by the lack of distinction between patients with and without CD in other regions of the bowel, short clinical follow-up periods, and small numbers of patients. We report on the clinical outcome and histology of 29 patients who had sigmoid resection specimens with diverticulitis and CD-like changes. Of the 25 patients who had no prior or concurrent CD at the time of surgery, 23 remained free of CD during the follow-up period (median, 6.0 yrs) and two developed CD in other regions of the bowel. All four patients with CD prior to their sigmoid resection continued to have active CD postoperatively. There were no histologic features of the sigmoid resection specimens that could be associated with the outcome of the patient. These results suggest that CD-like changes within the sigmoid resection specimen are an idiosyncratic inflammatory response to the diverticulosis rather than coexistent CD in the overwhelming majority of patients who do not have prior or concurrent CD at the time of sigmoid resection. Pathologists should be wary about making the diagnosis of sigmoid CD in the context of diverticulitis unless there is CD in other parts of the bowel.