The prevalence of curve progression was evaluated in 210 boys who had idiopathic scoliosis. A minimum age of eight years, a deformity of at least 10 degrees, and radiographic follow-up of one year or progression of the curve within the first year of follow-up were the criteria for inclusion in the study. Of the 210 patients, sixty-eight (32 per cent) had progression of 10 degrees or more. Four of the five patients who had had an initial curve of 50 degrees or more subsequently had a spinal arthrodesis. The risk of progression was significantly greater for patients who were at an earlier Risser stage (p < 0.002) and for those who were younger (p < 0.005). The risk of progression was also greater for patients who had had a larger curve at the time of presentation; of the sixty-three boys for whom the Risser grade was 1, 2, 3, 4, or 5 when they were first seen and who had a curve of 25 degrees or more, twenty (32 per cent) had progression, compared with only two (5 per cent) of the thirty-eight who had a Risser grade of 1, 2, 3, 4, or 5 and a curve of 24 degrees or less. Of the thirty-four patients for whom the Risser grade was 4 when they were first seen, five (15 per cent) had progression.(ABSTRACT TRUNCATED AT 250 WORDS)