Nasal continuous positive airway pressure (nCPAP) has been widely established in the treatment of obstructive sleep apnea syndrome (OSAS). However, only few studies have evaluated long-term effects of this treatment on lung function. This study assesses the effect of nCPAP on lung function parameters and response to bronchodilators in 50 OSAS patients. Spirometry and arterial blood gas measurements were performed before starting nCPAP and after 16.8±8 months of treatment. Of the 50 study patients (55±12 years, with an apnea/hypopnea index of 47±34h⁻¹), 15 had asthma, 13 had chronic obstructive pulmonary disease (COPD) and 22 had no obstructive airway disease (NOAD). In the entire population, significant decreases in FEF₅₀ (from 69±38% to 61±30%, P<0.005), FEF₂₅ (from 53±34% to 46±28%, P<0.05) and FEF₂₅₋₇₅ (from 65±33% to 57±27%, P<0.005) were observed after treatment. No impairment of lung function was found in COPD and asthmatic patients. In contrast, lung function was changed in the NOAD group where FEF₅₀, FEF₂₅ and FEF₂₅₋₇₅ as well as FEV₁ and FEV₁/VC ratio were significantly reduced. Moreover, bronchial hyperresponsiveness occurred in five of 22 patients of this group. These results suggest that tolerance of nCPAP should be handled by long-term follow-up of flow-volume loops.