The influence of the maxillary sinuses in acoustic rhinometry (AR) has not been evaluated, and this is the aim of the present study. We examined six subjects with AR and magnetic resonance imaging (MRI) after nasal decongestion to compare the area-distance relationships determined by the two methods. From the MRI data we obtained copies of the nasal cavities with and without maxillary sinuses, which were made in plastic by a stereolithographic method. AR curves from models without maxillary sinuses differed from AR curves with sinuses included but were in agreement with MRI curves without inclusion of sinuses. A similar difference in AR was seen in two subjects before and after the nasal cavities were flushed with saline to fill up the maxillary sinuses. The measured volume in the first 50 mm of the nasal cavity models was unaffected by the sinuses, but the volume in the first 70 mm corresponding to the length of the nasal cavity septum was increased slightly but significantly (from 10.8 to 11.3 cm3; P = 0.05). The presence of maxillary sinuses increased the volume of the epipharynx (70-100 mm from the nostril) from 12.2 to 21.3 cm3 (P < 0.01), and this increase was not due to the influence from the contralateral nasal cavity. We conclude that the maxillary sinuses may significantly contribute to the acoustically determined areas in the posterior part of the nasal cavity and the epipharynx, especially during decongestion, and may explain a part of the difference between area-distance curves obtained by AR and MRI, whereas contribution from the contralateral nasal cavity does not.