One hundred and thirty-three curves in 102 patients who were followed for an average of 40.5 years were evaluated to quantitate curve progression after skeletal maturity and for prognostic factors leading to curve progression. Sixty-eight per cent of the curves progressed after skeletal maturity. In general, curves that were less than 30 degrees at skeletal maturity tended not to progress regardless of curve pattern. In thoracic curves the Cobb angle, apical vertebral rotation, and the Mehta angle were important prognostic factors. In lumbar curves the degree of apical vertebral rotation, the Cobb angle, the direction of the curve, and the relationship of the fifth lumbar vertebra to the intercrest line were of prognostic value. Translatory shifts played an important role in curve progression. Curves that measured between 50 and 75 degrees at skeletal maturity, particularly thoracic curves, progressed the most.