The standing Cobb measure in a group of 287 girls with idiopathic scoliosis at the start of treatment was in the mean approximately 9° larger than the supine Cobb measure. This mean standing-supine difference was essentially independent of curve severity expressed by the supine Cobb measure. Because of this, when the difference was expressed as a percentage of the supine Cobb measure, the mean values of this ratio decreased substantially with increasing curve severity. The mean difference decreased only slightly with age through adolescence, indicating that spine lateral curves stiffen little as patients mature. The standard deviations in the standing minus supine Cobb differences were approximately 6° in the majority of the age and severity groupings studied. Ninety-three percent of these patients had standing-supine Cobb measure differences that were between 0° and 20°.