Objectives-Currently, radiography with measurement of the Cobb angle is still considered the reference standard for diagnosing scoliosis. However, the ionizing radiation hazard is drawing wide attention. Can 3-dimensional (3D) ultrasound (US) be an alternative modality for diagnosing and monitoring patients with scoliosis? The aim of our study was to assess the reliability and validity of 3D US imaging in the evaluation of scoliosis. Methods-A commercially available ultrasound system with a magnetic tracking system was selected for long-distance 3D US imaging. Straight phantoms and curved phantoms were scanned with the imaging system to evaluate the stability of the system for curvature measurements. Eight healthy adult volunteers and 28 patients with scoliosis were recruited for long-distance 3D US imaging. The intraclass correlation coefficient was used to test the reproducibility of the interobserver and intraobserver measurements for both the healthy adults and patients with scoliosis. A linear regression analysis and Bland-Altman plot were used to analyze the correlation and to determine the extent of agreement between the angles measured on US images and the Cobb angles measured on conventional radiographs. Results-The 28 patients with scoliosis included 10 male and 18 female patients aged 8 to 37 years (mean age +/- SD, 17.7 +/- 1.4 years; body mass index, <25 kg/m(2)). In the phantom study, there was no statistically significant difference between the angles measured by the 3D US imaging system and those measured by an angle gauge (P > 0.05). In the clinical study, there was very good interobserver and intraobserver reliability (intraclass correlation coefficients, >0.90) for the US imaging system, with a high correlation (r(2) = 0.92) and agreement between the US and radiographic methods. Conclusions-The long-distance 3D US imaging system offers a viable modality for diagnosing and monitoring scoliosis without radiation.