The aim of the present study was to investigate the effect of a mandibular advancement device (MAD) for the treatment of sleep apnea (SA) on plasma brain natriuretic peptide (BNP), left ventricular ejection fraction (LVEF), and health-related qualify of life (HRQL) in patients with mild to moderate stable congestive heart failure (CHF). Seventeen male patients aged 68.4±5.5 with an apnea–hypopnea index (AHI) ≥10 were equipped with an individually fitted MAD. SA was evaluated using a portable respiratory multirecording system before and after the initiation of treatment. Eleven patients completed follow-up and were evaluated after 6 months of treatment. The AHI reduced from 25.4±10.3 to 16.5±10.0 (p=0.033) compared to baseline and mean plasma BNP levels decreased from 195.8±180.5 pg/ml to 148.1±139.9pg/ml (p=0.035). SA-related symptoms, e.g., excessive daytime sleepiness, were also reduced (p=0.003). LVEF and HRQL were unchanged. We conclude that SA treatment with a MAD on patients with mild to moderate stable CHF appears to result in the reduction of plasma BNP levels. Further studies to investigate if the observed reduction in BNP concentrations also result in improved prognosis are warranted.