Aims The aim of this study is to validate a diagnosis of heart failure (HF) according to the European Society of Cardiology (ESC) guidelines among patients hospitalized at Sahlgrenska University Hospital, Gothenburg, Sweden, between 2000 and 2012. Methods and results In Sweden, it is mandatory to report all hospital discharge diagnoses to the Swedish national inpatient register. In total, 27 517 patients were diagnosed with HF at the Sahlgrenska University hospital between 2000 and 2012. Altogether, 1100 records with a primary (n = 550) or contributory (n = 550) diagnosis of HF were randomly selected. The diagnosis was validated according to the ESC guidelines from 1995, 2001, 2005, and 2008, and cases were divided into three groups: definite, probable, and miscoded. In total, 965 cases were validated, while 135 records were excluded for various reasons. Of the 965 records, the diagnosis was validated as definite in 601 (62.3%) and as probable in 310 (32.1%); only 54 (5.6%) of cases had been miscoded. Echocardiography, as an objective evidence of cardiac dysfunction, had been performed in 581 (96.7%) of the definite, 106 (34.2%) of the probable, and 31 (57.4%) of the miscoded cases. Among the probable cases, the main reason they had not been classified as a definitive diagnosis of HF was lack of examination by echocardiography (63.8%). Conclusions The overall validity of HF diagnosis at Sahlgrenska University Hospital is high. This may reflect a high diagnostic validity at the time of diagnosis in the national Swedish patient register, supporting the continued use of this register in epidemiological research.