The treatment success of nasal continuous positive airway pressure (nCPAP)-therapy is dependent, on the one hand, on the achieved relief of complaints and, on the other hand, on long-term patient compliance and avoidance of compliance-related complaints. Next to the problem of mask application, nasal complaints comprise the most frequently reported side-effects and are among the primary factors causing nCPAP-therapy to be discontinued prematurely. To assess the morphological changes in the nasal mucosa during nCPAP-therapy, we excised specimens of nasal mucosa tissue from 10 patients with obstructive sleep apnoea syndrome (OSAS) before and 3-10 months after establishing nCPAP-mask compliance. The specimens were examined by electron microscopy. In all these patients compliance with the CPAP-mask marked the initial part of therapy. In addition, mucociliary clearance was assessed by the saccharin test before and after therapy. In all patients the nasal epithelium underwent fundamental changes upon CPAP-therapy, which became manifest as modifications in the shape of epithelial cells, conglutination and clumping of the microvilli, and the appearance of immunocompetent cells. Once patients were nCPAP-mask compliant, mucociliary clearance was distinctly prolonged in all cases. A successful therapeutic concept should provide normalization of room temperature and air humidity once nCPAP-mask compliance has been achieved and include regular assessment of the condition of the mucosa in the upper respiratory tract. Only by these measures can nasal complications be countered or given therapy at an early stage.