Abstract Ankylosed teeth provide satisfactory fixed points from which to measure movement of the adjacent teeth. Forward movement of teeth was assessed on plaster casts of 30 subjects with unilateral ankylosis of a deciduous molar in one jaw. A control group and the opposing unaffected arches of the experimental group showed good symmetry. Forward movement of teeth was assessed from recordings of the difference in length between the affected and unaffected sides of the arch. In 22 of the 30 cases, the ankylosed side was longer. In 9, the longer ankylosed quadrant was associated with spacing of the anterior teeth either in each interdental space or as a large space mesial to the ankylosed tooth. Four showed imbrication of the incisors on the unaffected side alone. Forward movement of the first molars occurs in normal jaws and may be due to at least two factors; one may be to compensate for interdental attrition. The second is, as yet, unexplained but is not dependent on a forward drive from the posterior teeth. Tongue pressure may be concerned.