The Perdriolle torsionmeter assesses vertebral rotation on a spinal radiograph. It is frequently used to measure improvement in spinal derotation following Cotrel-Dubousset instrumentation for scoliosis. In this study, intraobserver and interobserver measurement error was examined during use of the torsionmeter. Intraobserver error was as follows: 53% of the measurements were accurate to within 5 degrees, and 21% erred greater than 10 degrees. Error from the actual value averaged 6 degrees. Interobserver error was as follows: Among six observers, only one third of the radiographs had measurements within 5 degrees of each other. Another one third erred by more than 10 degrees. Because of this significant intraobserver and interobserver error, precise measurements of rotation using the torsionmeter cannot be expected. Efforts to quantify spinal derotation with the torsionmeter after Cotrel-Dubousset instrumentation may not be valid.