The automated health plan data and data from medical chart abstractions from eight large health maintenance organizations were used to evaluate the positive predictive values (PPVs) of the International Classification of Diseases, 9th revision (ICD-9) codes for cases of peptic ulcers and upper gastrointestinal bleeding. Overall, 207 of 884 cases of peptic ulcers and upper gastrointestinal bleeding (23%) were confirmed by surgery, endoscopy, X-ray, or autopsy. The PPVs were 66% for hospitalizations with codes for duodenal ulcer (ICD-9-CM 532), 61% for gastric/gastrojejunal ulcer (ICD-9-CM 531, 534), 1% for peptic ulcer (ICD-9-CM 533), and 9% for gastrointestinal hemorrhage (ICD-9-CM578). The overall and diagnostic category-specific PPVs were generally similar for the various HMOs. This study, using data from a large number of health plans located in different geographical regions, underscores the importance of evaluating the accuracy of the diagnoses from automated health plan databases.