The maxillary ostial function, measured as pressure transmission capacity, ostial airway resistance, equivalent ostial diameter during nasal breathing and with a constant artificial air-flow of 16.7 cm3 X S-1 into the sinus, was compared in three groups of patients. One group consisted of 7 sinuses in 6 patients with maxillary pain but without any signs of sinusitis; a second group of 6 sinuses in 6 patients with intramaxillary cysts; and a third group of 6 sinuses in 5 patients with chronic maxillary sinusitis. No impairment of the ostial function was found in groups one and two. In the third group an increased airway resistance of the ostia and small equivalent ostial diameters were found in both sitting and recumbent positions. With the pressure recording technique, the ostium can be judged as patent, non-patent, or partially obstructed, but no further grading can be done. With the pressure-flow technique the ostial function can be evaluated as ostial resistance or equivalent ostial diameters under 'physiological conditions' when the ostium is patent. With a constant artificial air-flow into the sinus the equivalent ostial diameter can be estimated even in non-patent ostia but the equivalent ostial diameters obtained do not always reflect the true functional state of the ostium.