Erythrocytes from patients with Duchenne muscular dystrophy (DMD) were studied in relation to the extracellular calcium ion concentration. The shape of fresh erythrocytes in DMD was normal and not affected by Ca++. When DMD erythrocytes were stored in buffer solution with Ca++, however, the rate of the shape change over time was enhanced compared with that of cells from normal controls. The calcium content of DMD erythrocytes stored in buffer solution with Ca++ and dextrose was also two and one-half times that of control cells. In contrast, when DMD erythrocytes were stored in buffer solution with dextrose but without Ca++, no differences from controls were observed. The adenosine triphosphate content of DMD erythrocytes stored in the buffer solution was not affected by Ca++ concentration.