Apical Root Resorption (ARR) of the maxillary incisors is an adverse effect in patients during treatment with orthodontic appliance. Limited number of prospective investigations on ARR in patients with Removable Orthodontic Appliance (ROA) is available and the reliability of assessments of ARR in periapical radiographs has not been evaluated. The aims were to investigate 1) Observer agreement regarding assessment of ARR in radiographs obtained during treatment with ROA in children and 2) The extent and severity of ARR in children during treatment with ROA. Material and Methods One hundred ten children, with Angle class II malocclusion or anterior cross bite suitable for treatment with ROA were consecutively recruited. Periapical radiographs of maxillary incisors were obtained before (T0) and after six month of treatment (T1). The contour of the roots was scored as 0, 1, 2, 3 or 4 according to a modified Levander and Malmgren index, by five observers. Score 0 represent normal root contour, score 4 severe root resorption. Inter- and intra-observer agreement was calculated as overall agreement and kappa values. Results In the 110 patients, 49 females, 61 males, the mean age was 10.6 years. Totally 439 roots were available for assessment. Kappa values for pair wise inter-observer agreement during treatment ranged between -0.01 and 0.27. Kappa values for intra-observer agreement ranged between -0.09 and 0.45 before treatment and -0.22 and 0.48 after treatment. Conclusion Reliability in assessing root morphology/resorption in periapical radiographs obtained in children during treatment with removable appliance is low. ARR seems to be of minor extent, however, the low reliability indicates difficulties in evaluating the severity and thereby the clinical importance in this young patient group.