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A prospective study was carried out of the incidence and natural history of adolescent idiopathic scoliosis in 26,947 students. Data were obtained on 1,122 students with idiopathic scoliosis. The incidence of idiopathic scoliosis was 4.5 per cent. The female-to-male ratio was 1.25:1.0 over-all, but the ratio varied directly with the severity of the curve--that is, 1:1 for curves of 6 to 10 degrees, and 5.4:1 for curves of more than 20 degrees. Progression of the curve was determined by a two-year follow-up of 603 patients. Progression was observed in 6.8 per cent of the students and in 15.4 per cent of the skeletally immature girls with scoliosis of more than 10 degrees at the initial examination. In 20 per cent of the skeletally immature children with curves of 20 degrees at the initial examination, there was no progression. Spontaneous improvement of the curve occurred in 3 per cent and was seen more frequently in curves milder than 11 degrees. Treatment was required in 2.75 students per 1,000 screened.