Thoracic ratios (TRs) were measured segmentally (T1-12) in the chest radiographs of 412 children aged 0-17 years attending hospital with minimal disorder or diseases (boys 193, girls 219). A new method for measuring TRs was used which calculates the width of the left hemithorax, the right hemithorax and the total thorax relative to T1-T12 distance. The data were analysed in 3 age groups--infancy, childhood and puberty, after the classification of Karlberg (1989). The findings are as follows. 1. The chest broadens from T1 to about T10-11. 2. Between infancy and childhood, relative to its length the chest narrows from above downwards and particularly in the lower chest (T5-12 average diminution, boys 9.5%, girls 9.8%). In the upper chest, the narrowing is more marked in girls than boys (T1-4 average diminution, boys 5.1%, girls 8.2%). 3. Between childhood and puberty, the girl's but not the boy's chest narrows further in its lower half (below T6 average diminution 3.3%). At T6 and above there is no detectable change in the relative width of the chest in either boys or girls. 4. The relative narrowing of the chest during growth appears to result from several mechanisms: (1) elevation of upper rib-vertebra angles (above 90 degrees); (2) drooping of lower rib-vertebra angles (below 90 degrees); and (3) linear rib growth being impaired relative to thoracic spinal growth in the lower ribcage (T6-12) of girls between childhood and puberty (Grivas et al. 1991 d). 5. The hypothesis is suggested that the relative narrowing of the lower chest with increasing age reduces the rotational inertia of the thorax in gait. There is a greater need for such reduction in girls because of the greater rotational inertia generated by the mass of their larger pelves. This hypothesis provides a mechanical explanation for the proportionate change in the girl's lung in the later stages of growth (Simon et al. 1972). 6. Developmentally, the left hemithorax is ahead of the right hemithorax in childhood. 7. Thoracic asymmetry favouring the right chest is found, and more so in puberty than childhood which is connected with the larger size of the thorax and lung in the adult. 8. The evidence suggests that hemithoracic development is caudocranial; this is consistent with an adaptation of the human ribcage to control spinal rotation and counterrotation when bipedal gait was acquired in evolution. 9. In progressive infantile idiopathic scoliosis, the upper chest is funnel-shaped.(ABSTRACT TRUNCATED AT 400 WORDS)