In scoliosis, the Cobb measure of curve severity has been recommended over the Ferguson method because it had greater magnitude and appeared more sensitive to changes during progression and after treatment. This study made comparisons between the Cobb and Ferguson measures in radiographs of patients with idiopathic scoliosis to test whether the methods were really different, and to compare their precision. In 138 observations of 77 untreated patients there was a very high correlation (R2 = 0.98) between Cobb and Ferguson angle, with Cobb angle averaging 1.35 times greater. For sequential measures (mean interval 10 months), the percent changes agreed closely (R2 = 0.5). The relationship between Cobb and Ferguson angles remained the same in measurements of 24 patients wearing a brace compared with the unbraced condition and in 18 patients measured before and after Harrington rod surgery. Repeated measurements were made by three observers with the apex and end vertebrae pre-marked and held constant. For Cobb angle, the greatest range of measurements on any film was 8 degrees (pooled SD = 1.3 degrees). For Ferguson angle the greatest range was also 8 degrees (pooled SD = 1.8 degrees). Ferguson angle was slightly more sensitive to incorrect selection of end vertebrae. It was concluded that both methods can be useful for measuring curve magnitude. Ferguson angle should be measured and then adjusted by multiplying it by 1.35 in situations where Cobb angle measurement is technically difficult or invalid. Ferguson angle is better suited to automated measurement.