Recent studies have demonstrated poor reliability of the King et al. classification of idiopathic scoliosis. The purpose of the present work was to determine whether the reliability of the King classification would be improved by employing unambiguous rules for classification encoded in a computer program. Thus the only possible source of variability in classifying a given radiograph would be variable landmark identification on the radiograph. Coordinates of the four corners of each thoracic and lumbar vertebra were obtained by digitizing radiographs, using the central sacral line to define the y-axis. A computer algorithm located curve apices as the most laterally deviated vertebra in a range of two above to two below a change in the sign of vertebral tilt. End vertebrae of each curve were located as those with the greatest tilt of the superior (proximal end vertebra) or inferior (distal end vertebra). In a lumbar curve, the apical vertebra was defined as ‘crossing the midline’ if all four corners laid on one side of the vertical central sacral line. Curves were defined as in Table 1 in King et al., except that curve flexibility data were not used. The algorithm was verified on the five examples in King et al. It then classified the six examples in Lenke et al. and Cummings et al. which were classified unreliably by human observers. The algorithm was also tested on 33 radiographs of 17 patients with AIS, 8 radiographs were repeat marked by two observers. The algorithm overcame accuracy and reliability problems, except in rare cases when it was borderline whether or not a lumbar curve crossed the midline, when the apex level was ambiguous, or when a Cobb angle was close to 10°.