Background: nCPAP-therapy is standard for patients with obstructive sleep apnea syndrome (OSAS). This study investigated, if nCPAP changed the patients lung function., Patients and Methods: Lung function of 228 OSAS patients without co-prevalent lung disease, who received nCPAP for the first time, was examined (whole body plethysmography, spirometry, and blood gas analysis). After 1, 2, and 3 years the results of the re-examined patients were compared with their individual starting results., Results: The results of the patients in the basic and in the follow-up examinations was within standard range. During reexamination, an improvement of inspiratory vital capacity was registered repetitively, especially in smokers. On the other hand, MEF 50 decreased within the first two years and FEV (1) %VC, but not FEV (1), decreased after 2 and 3 years of reexamination., Conclusions: Neither obstructive nor restrictive lung diseases were diagnosed under nCPAP-therapy within three years. This study with a retrospective design showed significant improvements of vital capacity (VC) in patients with OSAS, especially in smokers, under nCPAP-therapy. This increase can be explained by an effect of practice in the examination and by recruitment of formerly hypoventilated lung areas. As MEF 50 and FEV (1) %VC depend on VC, the decrease of both parameters may be caused by the increase of VC. In summary, nCPAP did not cause negative effects of lung function in the investigated patients over a period of 3 years.