Heated humidification of nasal continuous positive airway pressure (nCPAP) reduces upper airway symptoms and improves initial use in obstructive sleep apnoea syndrome (OSAS). The present study aimed to assess the effect of heated humidification of nCPAP on upper airway symptoms and initial use in obstructive sleep apnoea.This study was of a randomised, crossover design. Subjects with polysomnographically confirmed OSAS were randomised to 3 weeks nCPAP treatment with heated humidification (nCPAP-humid) or placebo humidification (nCPAP pl-humid). Objective and subjective nCPAP use, upper airway symptoms, and treatment satisfaction were compared.Thirty seven of 42 patients completed the protocol. nCPAP-humid reduced the frequency of adverse upper airway symptoms. nCPAP use over 3 weeks was greater with nCPAP-humid compared with nCPAP pl-humid. No difference was found between the treatment arms in terms of subjective treatment satisfaction or alertness.Heated humidification of nasal continuous positive airway pressure reduces upper airway symptoms and is associated with a small increase in initial use but not subjective sleepiness or treatment satisfaction. The results support the use of heated humidification as a strategy to reduce side-effects related to continuous positive airway pressure but not routine initial use. [8,9] and perceived lack of benefit [10]. nCPAP-related upper airway symptoms, including nasal congestion, dry nose or throat, sore throat and bleeding nose, affect between 30-50% of OSAS patients and limit long-term acceptance [5,8,10,11].Mouth leaks during nCPAP are common and offer a rational mechanism for the development of adverse upper airway symptoms as they cause unidirectional nasal airflow and progressive drying of the upper airway mucosa, release of inflammatory mediators, increased nasal mucosal blood flow and increased nasal resistance [12][13][14]. Heated humidification of inspired air attenuates the increase in mucosal blood flow and nasal resistance under experimental conditions [12,13] and adverse upper airway symptoms in OSAS patients chronically treated with nCPAP [9,15,16]. In a recent crossover study, MASSIE et al.[15] demonstrated a small but significant improvement in CPAP use with heated humidified nCPAP (nCPAP-humid) compared to a 2-week washout period but no difference compared to CPAP with a cold passover humidifier [15]. As this study did not utilise a placebo intervention, the outcome variables in both humidification (heated and cold passover) arms were subject to a potentially significant placebo effect. PEPIN et al. [11] reported no difference in side-effects between patients who used humidity and those who did not, but no information was given regarding the type of humidity or selection criteria for humidity treatment, making it difficult to draw a firm conclusion. The addition of heated humidification to CPAP therapy requires systematic evaluation given the additional cost, set-up and cleaning time. This study aimed to assess the effect of heated humidific...