The aim of this study was to evaluate the risk of root resorption during orthodontic treatment of patients with aplasia, and to analyse the relative importance of some anamnestic and treatment variables. The subjects comprised 68 orthodontically treated patients with 1-16 congenitally missing teeth. The age of the patients was 11-20 years (mean 15 years). All patients were treated with fixed edgewise appliances. The purpose of the orthodontic treatment varied: to create optimal conditions for prosthetic restorations or osseointegrated implants, or to achieve aesthetic and functional alignment of teeth in less severe cases. The degree of root resorption was assessed before and after treatment from intra-oral radiographs of the maxillary incisors using a scale of 0-4. In all, 186 maxillary incisors were evaluated. The degree of apical root resorption was significantly greater in cases of multiple aplasia (4-16 missing teeth) than in those with only one to three missing teeth. Root form, treatment time with rectangular wires and intermaxillary elastics, and total treatment time were significantly related to root resorption. Discriminant analysis disclosed that the following variables were the most important determinants of root resorption: number of missing teeth, root form, and time with rectangular archwires and intermaxillary elastics. It is concluded that there is a high risk of apical root resorption during orthodontic treatment in patients with multiple aplasia (four or more teeth), in particular in teeth with an abnormal root form and lengthy treatment with elastics and rectangular archwires.