Diagnosis related group (DRG) 129 consists exclusively of discharges having a principal diagnosis of International Classification of Diseases, Ninth Revision, Clinical Modification code 427.5 (cardiac arrest). It excludes patients with more specific diagnoses (eg, myocardial infarction and arrhythmia) or patients admitted for a different reason and who subsequently experience cardiac arrest. This study used a one-stage sample design to select all DRG 129 discharges from random hospitals, stratified by their annual number of DRG 129 bills. Using blinded techniques, medical records specialists reabstracted the International Classification of Diseases codes for 857 medical records. For the bills that were not coded DRG 129 on reabstraction, physicians classified the incorrect bills by clinical situation and reason for error. Diagnosis related group 129 had significantly higher rates of coding errors and upcoding than other DRGs. Of discharges erroneously billed to DRG 129, 42.1% of the patients entered the hospital for heart disease other than cardiac arrest and 55.2% died after entering the hospital for other diseases. Attending physicians need to distinguish between the "immediate cause" of death for the death certificate and the "principal diagnosis" for reimbursement purposes.