A thoracolumbar gibbus is an uncommon but potentially dangerous feature of achondroplasia. In a series of unselected South African Negro achondroplasts, nine out of 17 had an abnormality of this type. In contrast, only one girl out of 20 affected individuals of European or mixed ancestry had a gibbus. It is likely that the high frequency of this deformity in South African Negro achondroplasts is the result of the traditional practice by which an infant is carried on the mother's back, with the legs straddling her waist and the spine in flexion. Wedging on the vulnerable vertebral bodies in the Negro achondroplastic infant can probably be prevented by avoiding this custom.