A patient interface to deliver a flow of air at a positive pressure with respect to ambient air pressure to an entrance to the patient's airways including at least the entrance of a patient's nares while the patient is sleeping, to ameliorate sleep disordered breathing, the patient interface comprising: a seal-forming structure constructed and arranged to form a seal with a region of a patient's face surrounding the entrance to the patient's airways; a shell, wherein the shell and the seal-forming structure form at least a portion of a plenum chamber pressurizable to a therapeutic pressure, wherein the shell includes a first port into the plenum chamber, the first port configured to allow air to flow between the plenum chamber and ambient, and wherein the shell includes a passageway including a second port into the plenum chamber, the passageway configured to communicate with the flow of air at positive pressure; a pair of headgear tubes to supply the flow of air at positive pressure to the plenum chamber, wherein the shell includes two inlet ports configured to connect to respective ones of the pair of headgear tubes, wherein the passageway communicates the two inlet ports with the plenum chamber via the second port, and wherein the passageway and second port thereof forms the only passage for the flow of air at positive pressure into the plenum chamber; and an anti-asphyxia valve provided to the shell and configured to regulate flow through the first port and the second port to provide a flow path for pressurized air when pressure in the plenum chamber is above a predetermined magnitude and provide a breathable flow path when pressure in the plenum chamber is below the predetermined magnitude or not delivered, the AAV including only a single flap portion structured and arranged to regulate flow through the first port and the second port, wherein the flap portion is biased or pre-loaded to an activated position when pressure in the plenum chamber is below the predetermined magnitude or not delivered to uncover the first port so that a breathable flow of gas is allowed to pass along the breathable flow path that extends through the first port, and wherein the flap portion is configured to cover the second port in the activated position so that exhaled air from the patient is prevented from entering the passageway, and wherein the flap portion is deflected to a de-activated position when pressure in the plenum chamber is above the predetermined magnitude to uncover the second port so that the flow of air at positive pressure is allowed to pass along the flow path for pressurized air that extends through the second port, and wherein the flap portion is configured to cover the first port in the de-activated position to maintain the therapeutic pressure in the plenum chamber in use. a seal-forming structure constructed and arranged to form a seal with a region of a patient's face surrounding the entrance to the patient's airways; a shell, wherein the shell and the seal-forming structure form at least a portion of a plenum chamber pressurizable to a therapeutic pressure, wherein the shell includes a first port into the plenum chamber, the first port configured to allow air to flow between the plenum chamber and ambient, and wherein the shell includes a passageway including a second port into the plenum chamber, the passageway configured to communicate with the flow of air at positive pressure; a pair of headgear tubes to supply the flow of air at positive pressure to the plenum chamber, wherein the shell includes two inlet ports configured to connect to respective ones of the pair of headgear tubes, wherein the passageway communicates the two inlet ports with the plenum chamber via the second port, and wherein the passageway and second port thereof forms the only passage for the flow of air at positive pressure into the plenum chamber; and an anti-asphyxia valve provided to the shell and configured to regulate flow through the first port and the second port to provide a flow path for pressurized air when pressure in the plenum chamber is above a predetermined magnitude and provide a breathable flow path when pressure in the plenum chamber is below the predetermined magnitude or not delivered, the AAV including only a single flap portion structured and arranged to regulate flow through the first port and the second port, wherein the flap portion is biased or pre-loaded to an activated position when pressure in the plenum chamber is below the predetermined magnitude or not delivered to uncover the first port so that a breathable flow of gas is allowed to pass along the breathable flow path that extends through the first port, and wherein the flap portion is configured to cover the second port in the activated position so that exhaled air from the patient is prevented from entering the passageway, and wherein the flap portion is deflected to a de-activated position when pressure in the plenum chamber is above the predetermined magnitude to uncover the second port so that the flow of air at positive pressure is allowed to pass along the flow path for pressurized air that extends through the second port, and wherein the flap portion is configured to cover the first port in the de-activated position to maintain the therapeutic pressure in the plenum chamber in use.