The progression velocity of scoliosis was studied for 535 consecutive girls with untreated adolescent idiopathic scoliosis. At the first visit the mean age of the patients was 13.9 years (range 9-19 years), the mean magnitude of the major curves was 23.4 degrees (range 5-60 degrees), and that of the minor curves 16.4 degrees (range 0-49 degrees). The median follow-up time was 1.1 years (mean 1.4, range 0.5-6.3 years). The right thoracic curves were most progressive. In the major left thoracic and left lumbar curve types, initial minor curves progressed faster than major curves. The scoliosis progressed faster in patients with minor thoracic kyphosis. In these patients major right thoracic curves were more common. The progression of scoliosis was clearly dependent on the growth velocity, also, the greater the curve at the first visit, the greater was the progression. The progression was most notable with a growth velocity of >or=2 cm/year, at ages between 9 and 13 years, bone ages between 9 and 14 years, Risser signs 0-1, and between 0.5-2 years before menarche.