Summary. The enlargement of the frontal sinus has been analysed in a longitudinal study of 49 males and 47 females for whom a first lateral cephalogram was available at from 2 to 5 years of age for 88 subjects and from 6 to 11 years for 8 subjects. Thereafter the cephalo-grams were taken at approximately yearly intervals and in 28 subjects a last cephalogram was taken at 24 years or older. In only six subjects was enlargement of the sinus still proceeding at the time of the last cephalogram. The enlargement was assessed by a standardized measurement of the maximum vertical height of the sinus. The median age for the first appearance of the frontal sinus was 3.25 years for the boys and 4.58 years for the girls. It enlarged on average to 32.60 mm (SD 9.10) in the males and 26.60 mm (SD 7.50) in the females. The median age at which the main increase in size of the sinus ceased was 15.68 years for boys and 13.72 years for girls, thus suggesting that the enlargement of the frontal sinus, a mainly osteoclastic activity, follows very closely the trends for growth in bone lengths
