This article tests agreement between demographic, diagnostic, and procedural information from primary-care physicians' office records and Medicare Part B claims for Maryland Medicare beneficiaries. The extent of agreement depended on the category of information being compared. Demographics matched poorly, probably due to incomplete record samples. Important diagnoses were often missing from the medical record. When claims indicated presence of disease, the patient was likely to have the disease, but claims did not capture all people who have the disease. Additionally, many laboratory tests and procedures were missing from the primary-care record. The appropriate use of either of these data sources depends on the specific research question that is being asked.