The development of the computer-controlled microscope system described has made it possible to estimate accurately the contribution made to the muscle fibre population by both Type 1 and Type 2 fibres. This method is more convenient, and at least as accurate as manual methods. It is also possible to measure the size of the individual fibres using this system. The system has been applied to this study of the spinal muscles in scoliosis. The results show that there is a difference in the ratio of Type 1 to Type 2 fibres in the spinal muscles on the 2 sides of the curve. The difference is found only at the apex of the curve. The results demonstrate that there are more Type 2 fibres on the concavity of the curve at the level of the apex. This difference is due to a disparity in the number of each fibre type rather than a difference in their relative sizes. The imbalance of Type 1 fibres at the apex of the curve may provide a sustained muscle pull on the spine so causing a curve convex to that side.