The aim of this short-term, single-blind, randomized, crossover study was to evaluate night-time efficacy of and compliance and physiological responses to average volume assured pressure support (AVAPS) versus pressure support (PS) ventilation. AVAPS or PS ventilation was delivered to nine stable hypercapnic COPD patients via a mask over two 5-day periods during consecutive weeks. The Synchrony ventilator was used to deliver mask ventilation as follows: 8 ml/kg of ideal body weight (as the targeted inspiratory tidal volume) with IPAP ranging from EPAP up to 30 cmH(2)O for AVAPS, and the patient's highest tolerated IPAP level for PS. EPAP was set at the minimum level for both modalities and oxygen was added at a fixed inspiratory fraction to maintain SaO(2) above 90%. Arterial blood gases, comfort (by VAS), and perceived sleep efficiency (SE by questionnaire) were measured at baseline (T0) and after three (T1) and five (T2) nights over the two periods. Compliance to ventilation, as measured by mean usage (hours/night), was also recorded. Measurements were similar between the two treatment periods at baseline. PaCO(2) and comfort VAS similarly improved for both modalities, whereas the SE score significantly improved at T2 with AVAPS (from 5.1 +/- 2.0 to 4.1 +/- 2.2, P = 0.001) but not with PS (from 5.1 +/- 1.7 to 4.7 +/- 1.3, P = 0.219). No difference in mean usage was found between the two modalities. Mask AVAPS is as comfortable and effective as PS at reducing respiratory acidosis but produces better perceived sleep efficiency in stable hypercapnic COPD patients.