We reviewed the cases of 727 patients with idiopathic scoliosis in whom the initial curve measured from 5 to 29 degrees. The patients were followed either to the end of skeletal growth or until the curve progressed. One hundred and sixty-nine patients (23.2 per cent) showed progression of the curve. The incidence of curve progression was found to be related to the pattern and magnitude of the curve, the patient's age at presentation, the Risser sign, and the patient's menarchal status. We found no correlation between progression of the curve and the patient's sex, Harrington factor, rotational prominence, family history, or radiographic measurements. A progression factor was calculated using the three strongest correlations available at initial examination: the magnitude of the curve, the Risser sign, and the patient's chronological age. A graph and nomogram are presented that can serve as a guide for advising patients' families and for planning continuing care.