A kit for fitting a patient interface device (10) to the naris of a patient. The patient interface device employs a replaceable nasal pillow (46). The kit comprises a plurality of nasal pillows each having a characteristic that varies from one of the plurality of nasal pillows to another of the plurality of nasal pillows. Each of the nasal pillows comprises a base portion (62), a casing (65) and a fill material (66). The base portion is structured to be disposed on a portion of the patient interface device. The casing is coupled to the base in a manner which defines a sealed cavity therein. The casing includes an outer portion (65) structured to sealingly engage a naris of a patient and an inner portion (67). The fill material is disposed in the sealed cavity defined by the casing and the base.