Rasterstereography has been shown to be a reliable method for three- dimensional surface measurement of idiopathic scoliosis with Cobb angles up to 50°. In this study, 25 patients with severe idiopathic scolioses (Cobb angles 47°-92°) were examined before and after operation (VDS instrumentation). The a.p. radiographs were digitized according to the Drerup method . The similarity of rasterstereographic and radiometric data was quantified by the rms. differences between the rasterstereographic and radiographic curves of lateral deviation and vertebral or surface rotation. The average rms. differences were 5.8 mm for lateral deviation and 4.4° for vertebral and surface rotation, which is about 40% higher than for mild to medium scolioses.