This article assesses the validity of ambulatory administrative and encounter data and patient self-reported information compared with information contained in the ambulatory medical record for 17 chronic diseases. Using a sample of 213 adults (18 to 64 years old) and seniors (65 years and older) enrolled in a health maintenance organization and receiving care at multispecialty group practice in Minneapolis, Minnesota, sensitivity and specificity for claims and self-report were calculated for each chronic condition using the medical record as the criterion standard. The analysis was performed first by blinded review and then repeated with an unblinded review.