Aim: Disorders of dentition and gums are highly common in contemporary societies and its prevalence is increasing gradually. All psychological and social problems due to dentofacial esthetics and oral function and periodontal disease deterioration and dental caries has made orthodontic treatments important and considering that the relationship of molars is the key to occlusion, this study examines the occlusion relationship (1st molar- canine).
Materials and methods: 1800 students aged 9-10 from 20 regions of Tehran were randomly selected and studied using cross-sectional method. After excluding mentally retarded and aberrant ethnicities and those with previous orthodontic treatment from sample size, the remnants were examined as 5 groups (class I, class II, class III, different right and left, unidentifiable) based on Angel classification.
Results: 48.4% of subjects were identified to have class I occlusion, 25.6% class II, 6.7% class III, 15.9% had different right and left, and 3.3% had unidentifiable occlusion.
Conclusion: the prevalence of normal occlusion in this study was much lower compared with similar studies in other parts of the world and in Iranian literature as well. Malocclusion showed an ascending fashion; disagreement of class I and II malocclusions with similar Iranian and foreign studies seem to explain genetic background, type of diet, differences in instrument and measurements, and sample size and ethnicity and other factors.