Ultrastructural studies have shown plasmalemmal defects in a proportion of non-necrotic muscle fibers in Duchenne dystrophy, suggesting that intracellular calcium overloading may be an important mechanism of muscle fiber degeneration. To investigate this assumption, we studied the localization of calcium with the von Kóssa method, with alizarin red, and with glyoxalbis-(o-hydroxyanil) in serial, fresh-frozen sections of 114 biopsy specimens. The biopsy material included Duchenne dystrophy (24 cases), other dystrophies (27 cases), inflammatory myopathies (47 cases), and normal controls (11 cases). Counted in each specimen were every muscle fiber, the so-called large-dark fibers, and all calcium-positive fibers. Separate counts were made of the large-dark, necrotic, and other types of calcium-positive fibers. In Duchenne dystrophy, non-nectotic calcium-positive fibers occurred with a mean frequency of 4.83 percent. For all other groups, the corresponding value was 0.57 percent, with a range of 0.21 percent (normals) to 1.76 percent (scleroderma), p less than 0.001. Large-dark fibers were 12 times more common in Duchenne dystrophy than in all other cases. Forty-three percent of the fibers were calcium-positive in Duchenne dystrophy, whereas calcium-positive large-dark fibers were extremely rare in the other cases.