Limitation of mandibular movement is one of the cardinal signs of temporomandibular disorders. Temporomandibular Opening Index (TOI) is a new technique to determine the restricted mouth opening in temporomandibular joint dysfunction syndrome (TMD). The purpose of this study was to compare the TOI of Turkish children with and without TMD in primary, mixed, and permanent dentitions. In this study, a total of 270 children were evaluated. One hundred thirty-five (135) subjects had no signs or symptoms of TMD (Group 1), while the remaining 135 subjects did have signs and symptoms of TMD (Group 2). Forty-five (45) patients were selected for each dentition for two groups. "Maximum Voluntary Mouth Opening" (MVMO) values and TOI ranges were calculated. A Mann-Whitney U test and a Kruskal Wallis test were used to compare the data (p < 0.05). TOI showed statistical differences between all dentitions with and without TMD (p < 0.05). There were no significant differences between different dentitions or genders in Group 1 or Group 2 (p < 0.05).