Background Sleep-disordered breathing may adversely affect heart function, and thereby contribute to the progression of heart failure. A study was undertaken in patients with idiopathic cardiomyopathy to document the characteristics of sleep-disordered breathing. Methods and Results Thirty-five patients with a diagnosis of idiopathic cardiomyopathy, comprising 20 patients with dilated cardiomyopathy (DCM) and 15 patients with hypertrophic cardiomyopathy (HCM), underwent overnight polysomnography. Of these 35, 16 (80%) of the DCM patients and 7 (47%) of the HCM patients had sleep-disordered breathing. Central sleep apnea-hypopnea syndrome (CSAHS) was seen in 10 DCM patients, but not in the HCM patients, and obstructive sleep apnea-hypopnea syndrome (OSAHS) was seen in 6 DCM patients and 7 HCM patients. CSAHS was seen in DCM patients with a low left ventricular ejection fraction. HCM patients with OSAHS had a significantly greater body mass index (BMI) than those without OSAHS and CSAHS (27.6±3.8 vs 22.0±4.0 kg/m 2, p< 0.05). DCM patients with OSAHS had a larger BMI than those with CSAHS (29.3±5.8 vs 24.2±4.0 kg/m 2, p< 0.05) and those without OSAHS and CSAHS (29.3±5.8 vs 21.3±3.1 kg/m 2, p< 0.05). Conclusions Sleep-disordered breathing is common in patients with idiopathic cardiomyopathy; half of DCM patients had CSAHS, which was closely associated with obesity.(Circ J 2004; 68: 338-342)