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We used the Perdriolle method to measure the rotation of each vertebra from the seventh cervical to the fifth lumbar level, before and after posterior spinal arthrodesis and Harrington-Luque instrumentation in fifteen patients who had idiopathic scoliosis. The primary curve was corrected an average of 50 per cent and the secondary curve, an average of 73 per cent. Following the operation, the average number of vertebrae involved in the primary curve increased from seven (range, five to ten) to nine (range, seven to fifteen). The rotation of the apical vertebra of the primary curve increased 10 degrees in four patients and 5 degrees in two patients and decreased 5 degrees in one patient. The rotation of the apical vertebra of the secondary curve increased 10 degrees in three patients and 5 degrees in one patient and decreased 5 degrees in four patients. In seven patients, correction of the curve was associated with unexpected counter-rotation, beyond the levels of instrumentation, of three or more vertebral segments that had been in neutral position before the operation.