To quantitate the intrinsic error in measurement, fifty anteroposterior radiographs of patients who had scoliosis were each measured on six separate occasions by four orthopaedic surgeons using the Cobb method. For the first two measurements (Set I), each observer selected the end-vertebrae of the curve; for the next two measurements (Set II), the end-vertebrae were pre-selected and constant. The last two measurements (Set Ill) were obtained in the same manner as Set II, except that each examiner used the same protractor rather than the one that he carried with him. The pooled results of all four observers suggested that the 95 per cent confidence limit for intraobserver variability was 4.9 degrees for Set I, 3.8 degrees for Set II, and 2.8 degrees for Set III. The interobserver variability was 7.2 degrees for Set I and 6.3 degrees for Sets II and III. The mean angles differed significantly between observers, but the difference was smaller when the observers used the same protractor.