Platelets of patients with idiopathic scoliosis (IS) have been shown to have decreased capacity to aggregate and secrete in response to certain agonists. Similarities between the contractile protein system of platelets and muscle have made the platelets a popular model for muscle disease. We attempted to characterize the function and structure of myosin in platelets of IS patients. Blood was obtained from seven IS patients and seven matched non-scoliotic healthy controls. The mean Cobb angle measurement of the IS patients was 35.4 degrees with a mean Risser sign of 2.2. Washed platelets were isolated from the blood, and the contractile proteins from the membrane and the cytosol compartments were isolated and analyzed by two-dimensional peptide mapping. As previously reported (J Biol Chem 258:9290, 1983), peptide maps of normal platelets revealed that the heavy chain of myosin located in the platelet membrane lacks one major spot relative to the cytoplasmic myosin. In IS patients the cytoplasmic myosin lacks the same peptide that is missing in the membrane myosin of normal individuals. In addition, the ATPase specific activity of the cytoplasmic myosin from IS platelets was significantly lower compared with the activity of the cytoplasmic myosin from normal platelets. These results suggest the presence of a fundamental abnormality of IS platelet contractile proteins.