The results in seventy-nine adolescent patients (ninety-five curves) who had idiopathic scoliosis treated with the Wilmington brace are reported. The average follow-up was two years and six months (range, one to nine years). Before treatment, all of the patients had a curve that measured 20 to 39 degrees and a Risser sign of zero or 1. Although the magnitude of the curve was generally reduced by about 50 per cent with the initial application of the brace, a gradual loss of this initial improvement was observed both during active treatment and after the patient was weaned from the brace. Although twenty-seven (28 per cent) of the curves had progressed more than 5 degrees at follow-up, Lonstein and Carlson reported that the projected probability of progression of untreated 20 to 29-degree curves is 68 per cent. Thirty-six per cent of the thoracic curves, 16 per cent of the thoracolumbar and lumbar curves, and 28 per cent of the double major curves had progression of more than 5 degrees. However, only 11 per cent of the patients had a curve that progressed sufficiently to warrant fusion. Our findings indicate that the Wilmington brace favorably alters the natural history of 20 to 39-degree idiopathic curves.