An anti-asphyxia valve (14) adapted to, in use, be disposed between a patient (21) and means (10) to deliver a breathable gas to the patient (20). The valve (14) comprises a housing (18, 20) having an interior, at least one port (30) to provide fluid communication between the housing interior and atmosphere and a flap means (32) comprising a first portion (34) adapted for mounting to the housing (18, 20) and a second portion (35) adapted to flex. The second portion (35) flexes between a first biased open position allowing gas to pass from the housing interior through the port(s) (30) to atmosphere when the difference in gas pressure in the housing interior and atmosphere is below a predetermined operating threshold and a second forced closed position substantially occluding the port(s) (30) when the difference in gas pressure between the housing interior and atmosphere is substantially equal to or above the operating threshold.
