Correlated histochemical, phase and electron microscopic studies were employed in examining the eosinophils from a patient with acute eosinophilic leukemia. Numerous morphologic alterations were observed in the leukemic eosinophils and eosinophilic myelocytes. These alterations included asynchronous nuclear-cytoplasmic maturation; an increase in cell size; the formation of eosinophilic granules which vary markedly in number, size, contour, and density; and the presence of fibrillar formations in some of the leukemic cells. Histochemically, the major alterations observed in the leukemic cells were the extensive deposition of glycogen in the cytoplasm and the demonstration of increased phosphorylase activity in these cells. Other minor variations in the histochemical reactivity of the leukemic eosinophils also have been described. Histochemical procedures included technics for proteins, amino acids, carbohydrates, hydrolytic and oxidative enzyme activities.