We examined the incidence of sleep-apnea syndrome (SAS; 5 or more episodes of apnea/hypopnea in 1 hour) in 213 patients (152 male, 67.8 +/- 10.9 years) with various cardiac diseases by a modified sleep polygraph (morpheus; Teijin Pharma, Tokyo) from July 2005 to April 2007. Mild sleep disturbance was defined as 5< or = AHI<20, moderate sleep disturbance as 20< or = AHI<40, and severe sleep disturbance as 40< or = AHI. SAS was seen in 87.3% of the patients. This high incidence sharply contrasts with 7.5% reported in factory workers in Japan. Body mass index, though significant, was scarcely correlated with the severity of SAS (p<0.01). As sleep disturbance became severe, the proportion of an obstructive, central, and eventually mixed obstructive-central SAS increased. Although the overall severity was not different between different categories of cardiac diseases, obstructive-central SAS was seen far more frequent in congestive heart failure. Hypertension was closely associated with apnea/hypopnea. A tight correlation between SAS and various cardiac diseases was suggested.