Objective. To analyze clinical and instrumental characteristics of sleep disorders in children with cardiomyopathies (CMPs). Patients and methods. We performed retrospective analysis of clinical, laboratory, and instrumental parameters in 107 children with CMPs aged 2 to 17 years treated in the National Medical Research Center of Children's Health in 2018–2019. The study sample was formed in accordance with inclusion criteria (confirmed diagnosis of CMP with functional class I or II, NYHA or Ross R.D.) and exclusion criteria (age <2 years, other heart and vascular diseases). We enrolled 26 children with hypertrophic CMP, 63 children with dilated CMP, and 18 children with unclassified CMP. According to the signs of sleep disorders (from sleep questionnaires filled in by parents), we formed 3 groups: patients with no sleep disorders (n = 40), patients with symptoms of insomnia/parasomnia (n = 26), and patients with indirect and/or direct signs of sleep apnea syndrome (SAS). We analyzed patients’ complaints, as well as clinical, instrumental (liver ultrasound, echocardiography, Holter ECG), and laboratory (glucose, cholesterol, alanine aminotransferase, and aspartate aminotransferase in serum) parameters. Results. Sleep disorders were identified in 63% of children: 58% had signs of insomnia/parasomnia and 38% had signs of SAS. In contrast to the questionnaires, medical records had information about sleep disorders only in two cases. Medical records primarily contained complaints of fatigue and reduced tolerance to physical activity (73%), excessive sweating (23%), and shortness of breath (17%). Patients with SAS usually had more complaints (according to their medical records), and their complaints were more diverse, including abnormal blood pressure, cephalgia, palpitations, and syncope. Body mass index (BMI) (p = 0.001) and serum glucose (p = 0.001) were higher in children with SAS than in children with normal sleep. Even after the exclusion of BMI, glucose levels (although being within the reference range) were still significantly higher in the SAS group (p = 0.020). The QTc interval at the maximum heart rate (HR) (p = 0.018) in children with sleep disorders was longer and had a positive correlation with serum glucose level (r = 0.195, p = 0.052). The analysis of echocardiography parameters (excluding the diagnosis factor) showed a smaller diameter of the pulmonary artery (p = 0.058) in children with SAS and correlation between right atrial remodeling and the factor of sleep disorder in children with various forms of CMP (p = 0.040). Conclusion. The analysis of sleep questionnaires revealed sleep disorders in 63% of children with CMP, including insomnia/parasomnia (24%) and/or SAS (38%). The presence of SAS was associated with a substantial number and variety of subjective complaints. The signs of myocardial electrical instability (longer QTc interval at maximum heart rate), association between QTc and serum glucose level, specific features of remodeling of the heart and blood vessels in patients with sleep disorders, and, most importantly, SAS in children indicate the need for early detection and correction of sleep disorders (insomnia, parasomnia) and main causes of SAS, such as chronic diseases of the ENT organs, overweight, and obesity. Treatment of sleep disorders is very important in terms of prevention of complications, treatment and prognosis of cardiomyopathy in children, which will help to increase therapeutic efficacy. Key words: children, cardiomyopathy, comorbidity, sleep disorders, sleep apnea, sleep questionnaires