It was the purpose of this study to analyse the biomechanical behaviour of posterior teeth under headgear traction with neighbouring teeth in different eruption stages. For doing so a finite element (FE) model of the right part of a human maxilla was developed, based on an almost anatomically correct commercial surface data set of a human maxilla. The FE model included the first molar (M1) with surrounding tooth supporting structures as well as the second (M2) and third molars (M3) in different eruption stages. Calculations were performed, simulating the initial tooth mobility of the M1 under application of headgear forces. The numerical results were compared to a clinical transverse study of orthodontic patients undergoing headgear treatment. From 85 included patients, 41 patients had erupted M2 and/or M3, in the remaining 44 patients these teeth were not yet erupted. The interapproximal gap was measured between M1 and the second premolar (P2) with varying headgear loading and as a function of eruption stage of M2 and M3. Headgear forces on the M1 in the clinical and numerical studies ranged from 1.5 to 7.5 N. After adjustment of the material parameters of the PDL, simulated results deviated less than 5% from the clinical data. A larger deflection of the M1 was registered in treatment situations prior to the eruption of M2 and M3. In absence of M3, about 30% of the applied force was transferred from M1 to M2. The displacements of M1 were twice as large as those of M2. Concluding, the simulated biomechanical behaviour of teeth under headgear loading was in satisfactory congruence with the clinical measurements. Thus the clinical recommendation that the headgear treatment should start in an early stage, prior to eruption of the second and third molars (M2 and M3), is supported by the results of this biomechanical study.