Respiratory stability index (RSI), a semi-quantitative measure of respiratory instability, was found to reflect congestive and other clinical status of acutely decompensated heart failure in the PROST study. Given that the association between RSI and another important factors affecting respiration, such as peripheral oxygen saturation (SpO), and the influence of oxygen inhalation on this association were undetermined, and that the association between common sleep-disordered breathing (SDB) parameters and RSI was unknown, we performed a subanalysis using PROST data. Correlation analyses were performed to evaluate the relationships between RSI, SpO, and other SDB parameters (3% oxygen desaturation index [3%ODI], respiratory disturbance index [RDI]) using Spearman's rank correlation. RSI and overnight mean SpO were not significantly correlated either after admission (n=38) or before discharge (n=36; r=0.27, P=0.10 and r=0.05, P=0.76, respectively). This correlation was also not affected by presence or absence of oxygen inhalation. 3%ODI, RDI and RSI were significantly and inversely correlated both after admission and before discharge. RSI and blood oxygen level were not significantly correlated irrespective of oxygen inhalation, while the SDB parameters were significantly correlated, suggesting that RSI reflects lung congestion independently of blood oxygen concentration and, thus, can be a useful indicator of the non-invasive assessment of lung congestion.