A retrospective study was conducted on 262 patients with minor idiopathic scoliosis, having supine angles of less than 30° and rib humps of less than 30 mm at the first examination. The initial radiographic measurement of the Cobb supine angle and rib hump height, recorded during a bending test performed on subjects in a sitting position, was correlated with the rate of scoliosis progression. Scoliosis progression was estimated graphically from an angle diagram established by monitoring untreated patients for 7 months to several years, depending on the rate of progression before treatment. More than 95% of the patients with initial supine angles of more than 17° or rib humps greater than 11 mm suffered from progressive scoliosis. The importance of the size of the supine angle, rib-hump height, scoliotic pattern, and state of maturation for the progression rate are analyzed. Knowledge of these parameters can be used to make individual prognoses for approximately 95% of these subjects.