In rats with experimental ischemic myopathy, there was a significant correlation (r = 0.778, p < 0.001) between muscle uptake of techne tium-99m (99mTc) diphosphonate and tissue calcium concentration. In addition, the accumulation of both calcium and 99mTc-diphosphonate in acutely injured muscles was further increased in rats with vitamin D-induced hypercalcemia. Histologic studies demonstrated staining of damaged muscle fibers with alizarin red, indicating the presence of microcrystalline or ultramicrocrystalline calcium salts. Staining of muscle fibers was most intense in the outer marginal zones of individual microscopic infarcts. Our results suggest that the uptake of99mTc-diphosphonate in acutely damaged skeletal muscle is directly related to the deposition of calcium salts within the injured muscle fibers.