We reviewed the medical records and radiographs of 109 consecutive patients who had juvenile idiopathic scoliosis. The sixty-seven girls and forty-two boys were a mean of six years and ten months old (range, three years and four months to nine years and eleven months old) when the curve was recognized. One hundred and four patients had a progressive curve: twenty-eight had a single mid-thoracic curve with the apex usually at the eighth thoracic vertebra (Group 1A), twenty-nine had a major mid-thoracic curve with the apex usually at the eighth thoracic vertebra and a secondary minor lumbar curve (Group 1B), twenty-seven had a single thoracic curve with the apex usually at the ninth or tenth thoracic level (Group 2), eight had a single thoracolumbar curve with the apex at the twelfth thoracic level (Group 3), and twelve had a major lumbar curve with the apex at the second or third lumbar level and a secondary minor thoracic curve (Group 4). Five patients (5 per cent) had a resolving curve.Eighty-nine of the patients who had a progressive curve were followed to skeletal maturity. Eighty-eight patients were managed with a brace. The curve progressed at a rate of 1 to 3 degrees per year before the age of ten years and 4.5 to 11 degrees per year after the age of ten years. In sixty-seven of the eighty-four patients in Groups 1 and 2, a spinal arthrodesis was performed before the age of fifteen years (mean age, eleven years and ten months; range, nine years and three months to fourteen years and eight months), at which time the mean curve was 47 degrees (range, 24 to 90 degrees). Eight patients were not seen by us until they were fifteen years old or more, at which time the thoracic curve was 74 to 120 degrees; a spinal arthrodesis was done in seven. The curves in Groups 3 and 4 had a more benign prognosis, and only three patients in these two groups had an arthrodesis.The pattern of the final curve was not always apparent at an early stage, and there was extension of the primary curve or development of secondary structural curves with progression. The chief prognostic feature at an early stage was the level of the most rotated vertebra at the apex of the primary curve, and the final pattern of deformity was defined by the level of the caudad neutral vertebra in the primary thoracic curve.