To investigate the possible influence of gravity on daily variations of curve magnitude in adolescent idiopathic scoliosis, standardized anteroposterior standing radiographs of the spine were taken at 8:00 AM and repeated in the same conditions at 8:00 PM on the same day, 19 girls with idiopathic scoliosis, aged 10-16 years were included. The Cobb angle of the primary thoracic curve was measured blindly by four skilled orthopaedic surgeons. The mean interobserver variation in measuring Cobb angles on the 38 radiographs was 1.6 degrees. The average Cobb angle on morning radiographs was 60 degrees (range, 42-91 degrees) as compared to 65 degrees (range, 47-89 degrees) in the afternoon, a significant (P < 0.001) difference using a paired two-sided Student t test. It was concluded that there is a statistically and clinically significant daily increase of curve severity in moderate to severe idiopathic scoliosis.