A patient with acute lymphoblastic leukemia presented with intense eosinophilia. Under the light microscope these eosinophils showed smaller eosinophilic granules and were detected as neutrophils by Coulter Gen-S cell counter. This counter identifies cell morphology by size and forward and right angle light scatter of cells. Under electron microscopy these eosinophils had smaller and fewer granules and very few crystalloid structures, thereby explaining the inability of the cell counter to identify them as eosinophils. Eosinophilia subsided at 6 months of treatment, i.e. 5 months after the patient went into morphological remission; cytogenetic and bone marrow analyses revealed no abnormality.