Assessment of brace treatment for idiopathic scoliosis should be based on long-term follow-up results compared with those in untreated cases. Such comparison, however, is extremely difficult from an ethical standpoint. Therefore, we conducted a comparable follow-up study on three groups of patients divided according to how well the brace had been worn during the period of treatment. Two hundred and twenty-six adolescent idiopathic cases in which brace treatment had been initiated before skeletal maturity were studied, 170 of which were followed up after the termination of the treatment. In 131 single curve cases, those who had worn the brace well showed significantly less progression than those who had worn it poorly, while 38 double curve cases failed to show significant difference between the two groups. Approximately two thirds of the cases demonstrated no major progression whether the brace had been worn well or not. Development of a method to select cases based on prediction of curve progression is an important project in the near future.