Nasal problems are frequent at high continuous positive airway pressure (CPAP). We hypothesized that a reduction of the nasal resistance reduces CPAP and investigated the effect of a nasal valve dilator (Nozovent ®) on CPAP in patients with obstructive sleep apnea. In a randomized cross-over design Nozovent ® was inserted in 38 patients during one of two nights using AutoSet T ®. CPAP differences >1 cm H 2O were considered as clinically relevant. With Nozovent ® the median CPAP pressure was reduced from 8.6 cm H 2O to 8.0 H 2O ( P=0.023) in all patients, but the number of patients with a reduction of CPAP by 1 cm H 2O was not significant. The median CPAP level among 20 patients requiring a CPAP level of above 9 cm H 2O was reduced from 10.3 to 9.1 cm H 2O, P<0.05. A clinical improvement with Nozovent ® was seen in 10 of 20 patients requiring a pressure of above 9 cm H 2O compared with 4 of 18 patients who needed lower pressures, P=0.025. Nozovent ® reduces the CPAP level 1 cm H 2O in 50% of patients requiring a high pressure (>9 cm H 2O). Future studies should identify possible patients benefiting from a nasal dilator during CPAP therapy.