In this study, we documented the clinical features of patients who had obstructive sleep apnea (OSA) and coexisting asthma and assessed the safety of nocturnal nasal continuous positive airway pressure (nCPAP) therapy on the stability of asthma. Nine patients (8 men and 1 woman) with asthma and OSA confirmed on all-night sleep study were studied. All patients suffered from frequent nocturnal asthma attacks, resulting in hospitalizations and respiratory arrests in 3. All patients had symptoms of heavy snoring, nocturnal choking, frequent sleep arousals, and excessive daytime sleepiness. They recorded their daily peak expiratory flow rates (PEFR) in the mornings and evenings, before and after bronchodilator in three 2-wk periods consisting of control, nCPAP, and control. During the period of nCPAP therapy, all patients recorded improvement in their PEFR. The mean prebronchodilator and postbronchodilator PEFR for the 9 patients were significantly higher during nCPAP therapy than during both control periods. This study confirms that nCPAP therapy can be used safely in treating patients with OSA and coexisting asthma. Furthermore, nCPAP treatment improves the asthma control and, in particular, the nocturnal attacks in this group of patients. These results also suggest that recurrent upper airway obstruction and snoring may be important triggering mechanisms of nocturnal asthma attacks.