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 H2O were considered as clinically relevant. With Nozovent the median CPAP pressure was reduced from 8.6 cm H2O to 8.0 H2O (P = 0.023) in all patients, but the number of patients with a reduction of CPAP by 1 cm H2O was not significant. The median CPAP level among 20 patients requiring a CPAP level of above 9 cm H2O was reduced from 10.3 to 9.1 cm H2O, P < 0.05. A clinical improvement with Nozovent was seen in 10 of 20 patients requiring a pressure of above 9 cm H2O compared with 4 of 18 patients who needed lower pressures, P = 0.025. Nozovent reduces the CPAP level 1 cm H2O in 50% of patients requiring a high pressure (> 9 cm H2O). Future studies should identify possible patients benefiting from a nasal dilator during CPAP therapy.