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The obstructive sleep apnea syndrome (OSAS) is associated with cardiovascular abnormalities including left ventricular hypertrophy, left ventricular diastolic dysfunction, and endothelial dysfunction. The present study evaluated whether N-terminal pro-B-type natriuretic peptide (NT-proBNP) and peak oxygen consumption (peak VO(2)), both integral markers of cardiovascular function, are related to OSAS severity. In addition, we tested whether NT-proBNP levels depend on body composition in OSAS patients, similar to what has been reported in patients without OSAS. Eighty-nine patients with untreated OSAS underwent NT-proBNP measurement, dual X-ray absorptiometry, and cardiopulmonary exercise testing. In a representative subgroup (n = 32), transthoracic echocardiography was performed. The severity of OSAS was classified based on apnea-hypopnea index (AHI) values as mild (AHI 5-15 h(-1)), moderate (AHI 15-30 h(-1)), and severe (AHI >30 h(-1)). OSAS was mild in 19 (21%), moderate in 21 (24%), and severe in 49 (55%) patients. NT-proBNP levels did not differ among patients with mild [30 (10-57)], moderate [37 (14-55)], and severe [24 (13-49) pg/ml; p = 0.8] OSAS and were not related to body mass index (r = 0.07; p = 0.5), percent lean body mass (r = -0.17; p = 0.1), and percent fat mass (r = 0.18; p = 0.1). Percent predicted peak VO(2) was on average normal and did not differ among patients with mild (115 +/- 26), moderate (112 +/- 23), and severe OSAS (106 +/- 29%; p = 0.4). Body weight-indexed peak VO(2) did not differ among patients with mild (31.9 +/- 10.3), moderate (32.1 +/- 7.9), and severe OSAS (30.0 +/- 9.9 ml kg(-1) min(-1); p = 0.6) either. Lower NT-proBNP (beta = -0.2; p = 0.02) was independently but weakly associated with higher body weight-indexed peak VO(2). In the echocardiography subgroup, NT-proBNP was not significantly related to left ventricular mass index (r = 0.26; p = 0.2). In conclusion, NT-proBNP and peak VO(2) are not related to OSAS severity, and NT-proBNP poorly reflects left ventricular hypertrophy in OSAS. The lack of a relationship between NT-proBNP and OSAS severity is not due to a significant influence of body composition on NT-proBNP. There is an association between higher NT-proBNP and lower peak VO(2), indicating that NT-proBNP is a marker of cardiorespiratory fitness in patients with OSAS. However, the association is too weak to be clinically useful.