Osteoporosis is a disease characterized by the development of nontraumatic fractures, most commonly in the vertebrae of elderly women. Approximately 500,000 elderly women in the United States are newly diagnosed with vertebral fractures every year, as the compressive strength of the vertebra, mainly determined by the density of cancellous bone and its cross-sectional area, declines with age. A recent study in women suggested that a polymorphism in the Sp1 binding site of the collagen type I gene (COLIA1) was related to decreased vertebral bone mass and vertebral fractures. Determining the phenotypic trait(s) responsible for this relationship and whether this association is manifested in childhood would further define the structural basis for decreased bone mass and help identify children "at risk" for fractures later in life. We therefore studied the COLIA1 gene polymorphism and measurements of the size and the density of vertebral bone in 109 healthy, prepubertal girls. On average, 22 girls with the Ss genotype and one girl with the ss genotype had 6.7% and 49.4% lower cancellous bone density in the vertebrae than girls with the SS genotype. In contrast, there was no association between the size of the vertebrae and the COLIA1 genotypes.