Statement of Problem: In some cases, a difference in size between the right and left teeth, during band selection for first molars, is observed.

Purpose: The aim of this study was to measure the mesiodistal width and to compare the teeth on left and right sides of jaws in individuals with normal occlusion, according to the sex.

Materials and Methods: In this cross-sectional study, sixty students (30 males and 30 females), age ranging from 16-18 with the mean age of 17.3, were selected through cluster random sampling from Mashhad high schools. The mesiodistal width of 1440 teeth, from the distal of first molar on right side to the distal of first molar on the left, were measured on study casts. Data were analyzed using paired t-test and Independent sample t-test.

Results: In upper jaw, mean of the mesiodistal width of each tooth on the right side was less than its counterpart on the left side except for the right second premolar which its width was more than that of the left one although the differences were not statistically significant. In lower jaw, mean of the mesiodistal width of each tooth on right side was more than the same tooth on the left except for the canines and second premolars. The differences were not significant. Generally the mesiodistal width of each tooth in males was more than its counterpart in females with a specially significant difference for canines and first molars.

Conclusion: According to this study, there was a little difference in me siodistal width of teeth between the left and right sides. In upper jaw, the majority of teeth on the right side were smaller than the left ones whereas in lower jaw the right side ones were larger than the left ones. Therefore, dental asymmetry in both jaws, on the left and right sides, were opposite. Comparing males and females, each tooth in males was larger than the same one in females and especially for canines and first molars the difference was significant. As a result dental asymmetry should be considered in orthodontic treatments.