To determine the prevalence of thoracic scoliosis in patients 10-20 years old using radiographs as a screening tool, we studied 783 patients who obtained chest radiographs for nonspinal reasons. We measured thoracic curvature in coronal and sagittal planes; we determined whether age, sex, and sagittal curvature predicted the coronal curvature. The prevalence of scoliosis was 9.3%: 7.0% had curvatures 10°-20°; 1.0% had curvatures 20°-30°; and 1.3% had curvatures of at least 30°. Females (13%) presented larger prevalence of scoliosis than males (4.8%) (P<0.01), also with a larger proportion of larger curvatures. Sex and thoracic kyphosis were independent predictors of the coronal curvature, age was not.