1. The data in a case of fatal leukemia with predominant eosinophilia in the peripheral blood and bone marrow are presented; we believe that this case was one of eosinophilic leukemia.
2. During the period of observation, these eosinophils showed progesssive immaturity as the symptoms became more severe. Eventually this "left shift" became so marked that a large proportion of the cells were terminally myeloblasts in both the blood and the bone marrow.
3. Autopsy revealed invasion of many of the tissues and organs with these mature and immature eosinophil granulocytes and with myeloblasts.
ACKNOWLEDGMENT Our thanks for helpful criticism are particularly extended to Dr. Charles Doan and to Dr. William Dameshek.