Thirty patients with mild idiopathic scoliosis were reviewed between 7 and 17 years after spinal maturity. These patients were not treated surgically because they were cosmetically acceptable, and unlikely to progress further. Standardised radiographs taken at spinal maturity and at review were comparable, so any change represented true progression in a fully mature spine. There was an increase of lateral curvature in 60 per cent of the patients; this was small and always under 10 degrees. No change was seen in rotation. Lateral curves over 30 degrees, with rotation of 25 degrees or more, were found to be almost twice as likely to progress. It is suggested that caution should be exercised in leaving these curves untreated, and follow-up into early adult life is advisable.