This study tested the hypothesis that asthma can promote obstructive sleep apnea (OSA) by looking at the prevalence of OSA among patients with difficult-to-control asthma receiving long-term oral corticosteroid (CS) therapy and examined some possible etiological factors. The study design was a prospective cohort study and was conducted in the pulmonary outpatient clinic of a tertiary care center in Haifa, Israel. Twenty-two consecutive patients with severe unstable asthma, 14 on continuous and 8 on bursts of oral CS, in addition to their standard therapy for a mean of 8.9 +/- 3.3 years, underwent a night polysomnography in a sleep laboratory regardless of sleep complaints. A standard questionnaire was completed upon attending the sleep laboratory. The OSA was defined as respiratory disturbance index (RDI) of > or = 5 and typical complaints. The correlation between RDI to asthma and morphometric parameters was tested. All but one patient had OSA [95.5% prevalence], with mean RDI of 17.7 +/- 2.5. The RDI values were significantly higher in the continuous CS therapy subgroup (21.4 +/- 3.4 vs. 11.1 +/- 1.6, p < 0.05]. The study group had above normal neck circumferences and body mass index. The former increased by 12.1% +/- 3.1% % to 29.8% +/- 1% during the oral CS therapy interval but had no significant effect on RDI as a covariant. This study showed an unexpectedly high prevalence of OSA among patients with unstable asthma receiving long-term chronic or frequent burst of oral CS therapy. It may be assumed that prolonged and especially continuous oral CS therapy in asthma increases airway collapsibility.