Maxillary sinusitis may be diagnosed incorrectly as TMJ pain-dysfunction syndrome because of a similarity of signs and symptoms. Both conditions can manifest with headache, facial pain radiating to the ear and the maxillary teeth, preauricular pain, and pain in the buccal vestibule posterior and superior to the maxillary tuberosity. It can be concluded that (1) more consideration should be given to sinus disturbances as a differential diagnosis in TMJ pain-dysfunction syndrome, (2) it may be preferable to refer some patients with TMJ pain to a medical center where specialists in dentistry, otolaryngology, neurology, rheumatology, and psychiatry can evaluate the patient, and (3) TMJ pain-dysfunction syndrome should be evaluated and treated by a dentist experienced in management of this disorder.