Mild stages of heart failure might be difficult to diagnose in severely obese individuals with a body mass index (BMI) >40 kg/m(2). Measurement of the N-terminal fragment of pro-B-type natriuretic peptide (NT-proBNP) is feasible for detecting cardiac impairment. The aims of our study were to measure NT-proBNP in plasma of severely obese patients and to compare the results with results for patients in different stages of manifest cardiac dysfunction. In 61 severely obese individuals (median BMI, 43.2 kg/m(2)) and 96 nonobese patients with existing heart failure [classified into New York Heart Association (NYHA) classes I-IV], NT-proBNP was measured in the fasting condition. A medical history, physical examination, electrocardiography, blood chemistry, and chest x-ray were performed in the obese group. In addition, echocardiography was performed in the NYHA group. In obese individuals, NT-proBNP was increased to a median of 356 (interquartile range, 221-458) pmol/L [854 (530-1099) ng/L] and was comparable (P >0.05) to the median value for NYHA I patients {289 (258-451) pmol/L [694 (619-1082) ng/L]}, but was significantly lower than in the other NYHA groups (P <0.001 for each). The prognostic relevance of increased NT-proBNP for risk of developing cardiac insufficiency in severely obese patients needs to be further evaluated.