Four cases of de novo acute nonlymphocytic leukemia (ANLL) with early eosinophilic differentiation are described. The clinical course did not differ from that of the usual forms of ANLL. Morphologic and cytochemical features that can support this diagnosis are discussed. Particularly, the cyanide-resistant peroxidase stain appeared to be a specific marker of eosinophilic differentiation. Acute eosinophilic leukemia is a distinct entity, and this unusual subtype of ANLL can be set apart from other forms of ANLL characterized by hypereosinophilia.