BACKGROUND: A reduced mouth opening capacity may be one of the first clinical signs of pathological changes in the masticatory system. The aim of this retrospective cross-sectional study was to create age related percentiles for unassisted maximal mouth opening capacity (MOC) of healthy children. METHODS: All recordings of MOC as measured at the yearly dental examinations of school children in the city of Zurich, Switzerland, between August 2009 and August 2010 were extracted from the database. The program LMSchartMaker Pro Version 2.43, Huiqi Pan and Tim Cole, Medical Research Council, 1997--2010 was used to calculate age and sex related reference centiles. RESULTS: Records from 22[prime]060 dental examinations were found during the study period. In 1286 (5.8%) the maximal interincisal measurement was missing. Another 55 examinations were excluded because of missing data for sex (7), age at examination (11) or because the value was deemed to be pathologically low (37). Thus, a total of 20[prime]719 measurements (10[prime]060 girls, 10[prime]659 boys) were included in the analysis. The median age (range) was 9.9 years (3.3-18.3) for girls and 10.0 years (2.8-18.7) for boys. The mean MOC (range) was 45 mm (25--69) for girls and 45 mm (25--70) for boys. Age related percentiles were created for girls and boys separately, showing the 3rd, 10th, 25th, 50th, 75th, 90th, and 97th percentile from 3 through 18 years of age. CONCLUSIONS: In these 20[prime]719 unselected school children MOC increased with age but showed a wide range within children of the same age.