The nose and not the mouth should be used for breathing as the nose has better air conditioning capacity. When air is inhaled through the mouth it may dry and cool the respiratory mucosa, which can lead to bronchoconstriction in sensitive patients with asthma. By dilating the nostrils you can increase nasal breathing in most subjects. The aim of this study was to investigate whether sleeping with dilated nostrils reduces nocturnal asthma. At the Asthma and Allergy Research Centre, Gothenburg, 15 out-patients with nocturnal asthma were selected. Every other night for 10 nights the test subjects slept with the nasal dilator Nozovent which has been shown to increase the nasal air-flow and decrease the need for mouthbreathing. Every morning the patients self-reported on a form whether they had woken with asthma during the night or if they had had to take asthma medication. When sleeping with the nasal dilator the patients woke up with asthma on 17 of 75 nights as compared with 32 of 75 when sleeping without the device (p < 0.01). Reduced nocturnal asthma was observed by 12 patients and less need for asthma medication at night by 7. None of the patients noted any side-effects due to the device. In conclusion, the easy-to-use and cheap medical device, Nozovent, which mechanically dilates the nostrils and improves nasal breathing, can reduce nocturnal asthma.