A mask assembly (10) for use in a system delivering a flow of a treatment gas to the airway of a pediatric patient. The mask assembly includes a mask body (12) having an opening (13) adapted to receive a treatment gas supply and a flexible peripheral seal structure (20). The flexible seal structure includes a first side coupled to the mask body and an opposite second side having an opening adapted to receive the face of the pediatric patient such that the second side sealingly engages the perimeter of the pediatric patients face. The opening is defined by a height (Hl) and a width (Wl). The ratio of the height to width of the opening is about 0.83.
