There have been numerous hypotheses about the pathogenesis of idiopathic scoliosis, but it is still unclear. There are some reports that abnormalities of contractile proteins may play a role in the pathogenesis of idiopathic scoliosis. The purpose of this report is to study the quantitative abnormalities of contractile proteins in muscles and nonactivated and activated platelets, and to determine whether or not the abnormalities in contractile proteins may play a role in the pathogenesis of idiopathic scoliosis. The materials were 21 idiopathic scoliosis patients aged from 13 years to 28 years(average 19.2 years) and 20 persons aged from 17 years to 25 years(average 20.1 years) as a control group. The electrophoretic analysis(SDS-PAGE method) was done on platelets both unstimulated and stimulated with thrombin and also on proteins of paraspinal muscles and gluteus maximus of idiopathic scoliosis patient and paraspinal muscles of control group. The results are as follows. 1. The myosin/actin ratios of triton-insoluble fractions to paraspinal muscles in convex sides of main curvatures of scoliosis patients(1.69±0.81) were significantly decreased compared to those of concave sides(2.55±1.28), gluteus maximus muscles(2.56±1.70) and control group(2.61±1.01). 2. There were no significant differences between scoliosis group and control group in the actin/myosin ratios of triton-insoluble fractions of the platelets both nonactivated and activated by thrombin. In conclusion, abnormalities of contractile protein in paraspinal muscles of convex side may play a role in the pathogenesis of idiopathic scoliosis, rather than abnormalities of systemic contractile protein.