BACKGROUND Orthodontic treatment can result in root resorption. OBJECTIVE To determine if apical root resorption is related to the type of appliance used and/or the direction and amount of tooth movement. METHODS The pre- and post-treatment tooth lengths of the maxillary and mandibular first molars and incisors were measured on panoramic radiographs of 114 subjects. Associations between changes in the lengths of the incisors and positional changes in the upper and lower central incisors were determined for 45 subjects with pre- and post-treatment cephalometric radiographs. RESULTS Taking into account the gender and OPT machine seven statistically significant differences were found between the appliance groups for the lengths of teeth 31 and 41. Within the groups four teeth decreased significantly in length when the pre-adjusted appliance was used (teeth 12, 11, 21, 26) and four teeth when the Speed appliance was used (teeth 26, 31, 41, 42). Only tooth 31 was shorter when the Tip-Edge appliance was used. Lower incisors were significantly shorter post-treatment if the apices were moved close to the lingual cortex. CONCLUSIONS When panoramic radiographs are used to assess treatment-induced changes in the lengths of the incisors, apical resorption is only one factor that should be considered. The images of lower incisors proclined during treatment may be foreshortened and/or the apices may lie outside the focal plane: both may result in 'shorter' teeth post-treatment. Because of the confounding factors panoramic radiographs may not be a reliable method of determining apical root resorption.