Complex sleep apnea syndrome (Comp-SAS) is the term used to describe a specific form of sleep disordered breathing characterized by the arise of central sleep apnea in patient with previous obstructive sleep apnea syndrome (OSAS) treated with continuous positive airway pressure devices (CPAP). The mechanisms of its occurrence are not well understood, but partly it seems to be a consequence of increased carbon dioxide elimination under positive airway pressure treatment and related improvement of pulmonary ventilation. The prevalence of Comp-SAS ranges from 5% to 20% of OSAS patient getting CPAP therapy with no significant predictors in comparison with simple obstructive sleep apnea, but more likely to happened in older males with more severe OSAS and accompanying cardiovascular pathology such as ischemic heart disease, atrial fibrillation and heart failure. In most cases of Comp-SAS, central apnea events are transient and disappear after continuous CPAP therapy use for 1 to 2 months. Novel treatment options like adaptive servo-ventilation or BiPAP-ST are available for such non-responders to CPAP but contra-indicated to patients with systolic heart failure. From the other hand, still not clear is it mandatory to treat all affected individuals with Comp-SAS if the disease is uncomplicated and patient is asymptomatic.