Objectives: The aim of this study was to assess the effect of fixed orthodontic treatment on DMFT index and white spot lesion formation. Materials and Methods: Fifty six patients (Group A: 26 subjects; Group B: 30 subjects) who were undergoing fixed orthodontic treatment were invited to participate in this study. All patients in Group B were treated with a 0.018 inch slot preadjusted fixed orthodontic appliances. An examiner used the Gorelick index for assessment of white spot lesion (WSL) on the buccal surface of teeth before (T1) and after (T2) treatment. The DMFT index value was recorded for each subject at T1 and T2. The WHO (World Health Organization) criteria and bitewing radiographs were utilized to diagnose the carious status of the subjects. Kruskal-Wallis and Mann-Whitney U tests were used to compare the groups. Results: No differences was found between the groups in terms of increased DMFT. On the other hand, The mean increase in WSL percentage in Group B (%66,6) and Group A (%26,9), respectively (p=0,000). In Group B, gender not significantly related to the DMFT index and development of new WSL. The greatest prevalence of WSLs was found in the mandibular first molars (%28,8), followed by the mandibular second premolars (%23,2) and the maxillary lateral incisors (%19,2). Conclusions: A more effective oral care support should be provided to subjects and caries preventive procedures (fluor therapy) should be done.