The fundamental similarity between platelets and muscle, suggested the possiblity of a shared defect in idiopathic scoliosis, a genetic disease with lateral deformity of the spine in which there is an elevation of calicum concentration in muscles and platelets. A variety of platelet tests revealed the following abnormalities: (1) Electron microscopic x-ray analysis and x-ray fluorescence spectrometry showed a 2- to 3-fold increase in calcium and phosphorus in whole cells and in individual dense bodies. (2) Electron microscopy morphometry revealed an increase in electron-opaque bodies in air-dried cells; granules and microtubules were unchanged. There were more large cells and membranous complexes. (3) Aggregations with epinephrine and ADP were depressed in some patients. (4) Proteins (total and contractile) and myosin. ATPase activity in centrifuged fractions of platelets were decreased in the cytosol and increased in the fraction containing membranes and granules. The correlated findings suggest that platelets in idiopathic scoliosis have a mild calcium transport defect related to membrane and/or contractile protein metabolism. This investigation also shows that platelets may be used to advantage in diagnosis and research of muscle diseases.