In a prospective study by the Scoliosis Research Society, 286 girls who had adolescent idiopathic scoliosis, a thoracic or thoracolumbar curve of 25 to 35 degrees, and a mean age of twelve years and seven months (range, ten to fifteen years) were followed to determine the effect of treatment with observation only (129 patients), an underarm plastic brace (111 patients), and nighttime surface electrical stimulation (forty-six patients). Thirty- nine patients were lost to follow-up, leaving 247 (86 per cent) who were followed until maturity or who were dropped from the study because of failure of the assigned treatment. The end point of failure of treatment was defined as an increase in the curve of at least 6 degrees, from the time of the first roentgenogram, on two consecutive roentgenograms. As determined with use of this end point, treatment with a brace failed in seventeen of the 111 patients; observation only, in fifty-eight of the 129 patients; and electrical stimulation, in twenty-two of the forty-six patients. According to survivorship analysis, treatment with a brace was associated with a success rate of 74 per cent (95 per cent confidence interval, 52 to 84) at four years; observation only, with a success rate of 34 per cent (95 per cent confidence interval, 16 to 49); and electrical stimulation, with a success rate of 33 per cent (95 per cent confidence interval, 12 to 60). The thirty- nine patients who were lost to follow-up were included in the survivorship analysis for the time-period that they were in the study. Treatment with a brace was successful (p < 0.0001) in preventing 6 degrees of increase or more until the patients were sixteen years old. Even a worst-case analysis, in which the twenty-three patients who were dropped from the study after management with a brace were considered to have had failed treatment, showed that the brace prevented progression and that this effect was significant (p = 0.0005). There was no difference in the degree of increase in the curve between the patients who were managed with observation only and those who were managed with electrical stimulation.