Commercially available nasal masks have a large mask volume and give rise to considerable air leaks around the mask during nasal intermittent positive pressure ventilation (NIPPV) which may reduce alveolar ventilation (VA per breath). The effects of a custom-fabricated nasal mask (F-mask) versus a commercially available mask (C-mask) on arterial blood gas measurements, dead space including both physiological and apparatus dead space (VD), air leak and VA per breath were compared in patients with restrictive thoracic disease during short-term NIPPV sessions while using a volume cycled ventilator with equivalent settings for both masks. The mask volume of the C-mask was significantly larger than that of the F-mask (p<0.003). The arterial carbon dioxide tension (Pa,CO2) during NIPPV with either the F-mask (5.56+/-1.35 kPa) (mean+/-SD) or the C-mask (6.87+/-0.96 kPa) was significantly lower than during spontaneous breathing (7.75+/-0.81 kPa; p<0.003), but the Pa,CO2 decreased more during NIPPV with the F-mask than with the C-mask (p<0.003). The VD was significantly smaller (p<0.03), the air leak was significantly less (p<0.03), and the VA per breath was significantly larger (p<0.03) during NIPPV with the F-mask than with the C-mask. In conclusion, nasal intermittent positive pressure ventilation with the F-mask was more effective than nasal intermittent positive pressure ventilation with the commercially available mask due to its smaller dead space and less air leak. Further studies are needed to extend these results to all the commercially available-masks.