Heart failure (HF) costs are largely due to hospitalization. The validity of a death/discharge diagnosis of HF (DDHF) is largely unknown. The authors assessed the validity of DDHF and the impact of misdiagnosing. The case notes of patients consecutively admitted to a medical department between January and June 2001 were reviewed. Cases with DDHF or cardiovascular diseases, potential precursors of HF (PPHF), were included. The diagnosis of HF (European Society of Cardiology guidelines) was classified as definite, possible, or miscoded. Of the 1038 patients admitted, 234 were enrolled: 157 with DDHF and 77 with PPHF. One hundred eighty patients had a definite diagnosis of HF. Of the 157 diagnoses coded as definite HF, 130 were correct, 21 had possible HF, and 6 were miscoded. Of the 77 patients classified as having PPHF, 38 had definite HF. The accuracy of the DDHF diagnosis was 72.2%: 21.1% were underdiagnosed and 8.3% overdiagnosed. DDHF failed to capture many HF admissions and therefore alone underestimates the prevalence, burden, and costs of the syndrome.