Coronoid process locking has been regarded a rare condition. The literature reveals 59 reports each of one or two cases and an additional two papers reporting on four and six patients, respectively, the latter including hereditary cases. Coronoid process elongation tends to be overlooked when limitation of mouth-opening ability is investigated, since most interest is focused on the joint. To study the frequency of coronoid process hyperplasia and its possible association with TMJ disk displacement, 163 patients with limited mouth-opening ability were examined. All patients were referred for radiographic examination of the TMJs. In eight patients (5%), the limitation of mouth-opening ability was caused by elongation of the coronoid process. Four cases were of congenital origin, and four were secondary to long-standing disk displacement without reduction. The results of this study indicate that elongation of the coronoid process is more common than previously thought. In cases of limited mouth opening, attention should be paid to the possibility of coronoid process locking.