Etiology of Eosinophilia Eosinophilia may be Primary: A clonal proliferation of eosinophils associated with hematologic disorders such as leukemias and myeloproliferative neoplasms Secondary: Caused by or associated with nonhematologic disorders (see table Important Disorders and Treatments Associated With Eosinophilia) Idiopathic: Cause cannot be identified The most common cause of eosinophilia in the US is Allergic or atopic disorders (typically respiratory or dermatologic) Other common causes of eosinophilia include Infections (typically parasitic) Certain tumors (hematologic or solid, benign or malignant) Almost any parasitic invasion of tissues can elicit eosinophilia, but protozoa (amoeba) and noninvasive metazoa usually do not.