The size of maximal active movements of the mandible were studied in children in two age groups. The younger group consisted of 33 children, aged 1-2 years, in whom mouth opening was recorded using an indirect method. Highly significant differences were found between 3 recordings with the third recording having the largest mean, 38.4 mm.The older group consisted of 75 boys and 75 girls, aged 6 years. None of them had pain or severe symptoms of dysfunction of the masticatory system. The mean maximal opening was 44.8 mm. The means found for maximal lateral movement to the right and to the left and for maximal protrusion were each about 8 mm. No differences in vertical or horizontal movements were found between the sexes. The mobility of the mandible varied greatly inter-individually, but as the various movements were highly significantly correlated in both boys and girls, the value found for one movement must be evaluated in relation to the sizes of the other movements.The ranges within which a pair of observations of maximal mandibular movements in a healthy 6-year old child may be expected to fall with 95% probability were calculated and presented graphically. Repeated recordings of maximal opening are recommended for children.