Study Design.: Three experiments to validate the use of the flexible electrogoniometer (FEG) as a tool to measure thoracic kyphosis. Objective.: To investigate the accuracy, test-retest reliability, and concurrent validity of the FEG as applied to the thoracic spine. Summary of Background Data.: Thoracic kyphosis is commonly measured by the Cobb angle from lateral radiograph. Other less-invasive tools have been developed, but all yield only static measurements or are restricted to the laboratory. The FEG, which can record joint angles over time outside the laboratory, has been used to measure other joints but has not yet been validated for measurement of the thoracic spine. Methods.: First, the FEG was bench-tested against a plurimeter for accuracy. Second, 12 subjects performed 7 functional activities 1 week apart to assess the test-retest reliability. Finally, to examine concurrent validity, 12 subjects underwent radiography in "upright" and "slumped" standing with the FEG attached to the skin over their thoracic spine. Three Cobb angles, which corresponded with the inner, mid, and outer margins, respectively, of the overlying FEG end blocks were compared with the FEG angles. Results.: The correlation between the FEG and the plurimeter was excellent (r > 0.99, P < 0.0001), although some accuracy was lost at extremes of range. The mean correlation between the first and second measurements was very strong (intraclass correlation coefficient2,1 0.92, P < 0.0001; range, 0.89-0.95). The mid-Cobb angle showed the least absolute angular difference from, and was highly correlated with, the FEG angle (r = 0.81, P < 0.01). Conclusion.: The FEG demonstrated excellent accuracy and test-retest reliability and correlated very well with the Cobb angle. The FEG measurement seemed to correspond most closely with the Cobb angle measured between the middle of the FEG end blocks.