The authors evaluated the effect of patients' comorbidity on the appropriateness of performing esophagogastroduodenoscopy or cholecystectomy. A nine-member national physician panel rated 1,118 brief clinical scenarios for patients without comorbidity. Ratings were then repeated for patients with increasing degrees of comorbidity. As comorbidity changed from none to medium, 60 percent of those scenarios that were originally rated as appropriate for endoscopy and cholecystectomy remained appropriate. As high comorbidity was introduced, only 13 percent of such scenarios remained appropriate for endoscopy, while 33 percent remained appropriate for cholecystectomy. These findings suggest that, although clinical reasons for performing procedures are a powerful determinant of when they should be used, comorbidity is also important and needs to be included in any assessment of the appropriateness of procedure use.