In order to study the immediate 3-D effect of the Boston brace on lumbar scoliosis, 31 adolescents with idiopathic scoliosis King type I or II had a 3-D computerised reconstruction of their deformity with and without the brace. Results demonstrate that the brace produces a distraction of the lumbar spine similar to that produced by the Harrington instrumentation by correcting the frontal deformity at the expense of a significant reduction of the physiological lumbar lordosis. No significant effect on rotation of the apical vertebra or "detorsion" of the spine could be measured. We feel that a orthosis that provides a real 3-D correction of the deformity has yet to be developed.