Background: It has been shown that sleep-disordered breathing (SDB) is associated with adverse prognosis in patients with chronic heart failure (CHF), but little is known about the relationship between SDB and life-threatening arrhythmias. Methods and Results: Fifty patients with CHF and SDB (33 male; mean age, 61 years) underwent Holter electrocardiogram and portable sleep monitoring simultaneously. The circadian variation in positive T-wave alternans (TWA; >65μV) was determined during 6-h intervals (0–6, 6–12, 12–18, and 18–24h). In addition, power spectral analysis of heart rate variability (HRV) was evaluated across a 24-h period. The subjects were divided into 2 groups based on whether respiratory disturbance index was ≥20events/h (Group A, n=24) or not (Group B, n=26). The prevalence of positive TWA, parameters in HRV and the occurrence of ventricular tachycardia (>5 beats) were compared between the 2 groups. The prevalence of positive TWA in Group A was significantly higher than that in Group B in all 6-h intervals. Low-frequency and high-frequency powers of HRV were significantly lower in Group A than in Group B across a 24-h period. Importantly, the prevalence of ventricular tachycardia was significantly higher in Group A than in Group B (46% vs. 19%, P=0.04). Conclusions: SDB may induce cardiac electrical instability associated with life-threatening arrhythmias across a 24-h period in CHF.  (Circ J 2013; 77: 1466–1473)