Of the 8,973 patients with sleep related breathing disorders examined in our department, about 4% were found to have a hyperreagible bronchial system. Provocation tests were performed before and after a 3-day CPAP therapy in 8 male patients with confirmed bronchial hyperreagibility. Two of these 8 patients revealed a marked increase of the respiratory path resistance and a decrease in FEV1.0 of 30% in the provocation test after 3 days. This did not occur with the other patients. From these findings, it is concluded that CPAP therapy can lead to an increase in the hyperreagibility in some patients with hyperreagible bronchial systems. However, further investigations are needed to identify the underlying causal relationships.