How well can hospital discharge abstracts be used to estimate patient health status? This paper compares information on comorbidity obtained from hospital discharge abstracts for patients undergoing prostatectomy or cholecystectomy at a Winnipeg teaching hospital with clinical data on preoperative medical conditions prospectively collected during an Anesthesia Follow-up study. The diagnostic information on cardiovascular disease, respiratory disease, and metabolic disorders showed considerable agreement, ranging from 65 to over 90% correspondence across the two data sets. Certain conditions noted by the anesthesiologist were often absent from the claims data; cardiovascular disease was recorded in the clinical data but absent from the claims for 31% of prostatectomy and 17% of cholecystectomy cases. Such patients were less likely to have been assigned a high score on the ASA Physical Status measure or to have high-risk diagnoses on the hospital file. Similar findings resulted from comparing the two sources in their ability to predict such adverse outcomes as mortality and readmission to hospital: the anesthesia file generally included less serious comorbidity.