BACKGROUND: Clinical registries are used increasingly to analyze quality and outcomes, but the generalizability of findings from registries is unclear. METHODS: We linked data from the Acute Decompensated Heart Failure National Registry (ADHERE) to 100% fee-for-service Medicare claims data. We compared patient characteristics and inpatient mortality of linked and unlinked ADHERE hospitalizations; patient characteristics, readmission, and postdischarge mortality of linked ADHERE patients to a random 20% sample of Medicare beneficiaries hospitalized for heart failure; and characteristics of Medicare sites participating and not participating in ADHERE. RESULTS: Among 135,667 ADHERE records for eligible patients ≥ 65 years, we matched 104,808 (77.3%) records to fee-for-service Medicare claims, representing 82,074 patients. Linked hospitalizations were more likely than unlinked hospitalizations to involve women and white patients; there were no meaningful differences in other patient characteristics. In-hospital mortality was identical for linked and unlinked hospitalizations. In Medicare, ADHERE patients had slightly lower unadjusted mortality (4.4% vs 4.9% in-hospital, 11.2% vs 12.2% at 30 days, 36.0% vs 38.3% at 1 year [P < .001]) and all-cause readmission (22.1% vs 23.7% at 30 days, 65.8% vs 67.9% at 1 year [P < .001]). After risk adjustment, modest but statistically significant differences remained. ADHERE hospitals were more likely than non-ADHERE hospitals to be teaching hospitals, have higher volumes of heart failure discharges, and offer advanced cardiac services. CONCLUSION: Elderly patients in ADHERE are similar to Medicare beneficiaries hospitalized with heart failure. Differences related to selective enrollment in ADHERE hospitals and self-selection of participating hospitals are modest.