Aim: The aim of the Danish National Database for Obstructive Sleep Apnea (NDOSA) was to evaluate the clinical quality (diagnostic, treatment, and management) for obstructive sleep apnea and obesity hypoventilation syndrome in Denmark using a real-time national database reporting to the Danish National Patient Registry. Study population: All patients diagnosed with obstructive sleep apnea or obesity hypoventilation syndrome at public and private in- and out-hospital departments in Denmark were included. Main variables: The NDOSA contains information about baseline characteristics, comorbidity, diagnostic procedures conducted, treatment (continuous positive airway pressure and similar treatments, surgery, mandibular advancement devices, etc) complications, and treatment status. Descriptive data: Yearly report with indicators for diagnostic procedures, treatment, and follow-up comparing different departments was involved in the management of sleep apnea in Denmark for the purpose of quality improvement. Conclusion: The NDOSA has proven to be a real-time national database using diagnostic and treatment procedures reported to the Danish National Patient Registry.