The purpose of this study was to determine the extent of apical root resorption, after fixed appliance therapy, in the posterior part of the dentition, and to evaluate if predisposing factors can be pointed out retrospectively. Tooth length in the posterior part of the dentition was measured on pretreatment and posttreatment orthopantomograms of 153 patients who were treated with standard edgewise appliances. Pretreatment and posttreatment tooth lengths were determined, and an attempt was made to explain the differences by sex, age, stage of root formation at onset of orthodontic treatment, extraction versus nonextraction therapy and treatment duration. Patients were divided into two groups according to their root formation. Group A consisted of patients with incompleted root formation at onset of orthodontic treatment except for the first molars. Group B consisted of patients where root formation was completed with the exception of second and third molars. Posttreatment tooth lengths of groups A and B were compared with pretreatment tooth lengths of group B with a paired t test, and correlations with the variables were calculated with multiple regression analysis only in group B. Posttreatment group A showed no significant differences with the mean root lengths of pretreatment group B. Differences between the mean pretreatment and posttreatment root lengths in group B were significant. Posterior teeth showed root shortening during active orthodontic treatment independent of sex, age, extraction versus nonextraction therapy, and duration of active treatment. Teeth with incompleted root formation at onset of orthodontic treatment showed root lengthening during active treatment, yet did not reach their "normal" tooth length.