A nasal mask which is suitable for use in a continuous positive airway pressure system. The mask has a face contacting portion mounted to a shell which is sized and shaped to overfit the nose region of an intended wearer, and the face contacting portion is in the form of a distendable membrane which is moulded from an elastic plastics material. The distendable membrane and the shell together define a chamber, and pressurised gas admitted to the chamber causes the membrane to distend outwardly from the shell. When placed in contact with the face of the wearer, the distendable membrane is caused to overlay the covered facial regions and, under the influence of the pressurised gas, to conform three-dimensionally with the contours of the overlayed regions. An orifice is formed within the membrane and is shaped and positioned to admit gas from the chamber to the nasal passages of the wearer.