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Age changes in extent of costochondral ossification of the first rib and of the lower ribs were evaluated separately from chest roentgenograms in five populations: European Americans, Lebanese, Solomon Islanders (the Lau and the Baegu), and a special veterans group. Increase in the ossification was closely associated with age in all groups. The shapes of the age curves were similar in all populations within each measure and within sexes. However, the Solomon Islanders showed less ossification than the Caucasians, and the Baegu showed less ossification than the Lau. These findings may be explained by the dietary differences in the populations. With respect to sex differences, for the first rib, males showed greater ossification than females regardless of age in each of the groups. For the lower ribs, males generally showed most age changes before age 45 and females after age 45. The sex differences may be related to endocrine factors. Ossification in the first rib cartilage was related to chest circumference in all three male groups investigated (the veterans, the Lau and the Baegu) but not in the females (the Lau and the Baegu). Ossification in the lower rib cartilages was related to chest expansion in the male veterans, the only group where such data were available. These latter findings supported the hypothesis that biomechanical factors influence costochondral ossification.