A 79-year-old man with persistent eosinophilia and elevated immunoglobulin E.

The differential diagnoses for eosinophilia include allergic, infectious, autoimmune, and neoplastic diseases. We presented the case of a 79-year-old man with eosinophilia and elevated immunoglobulin E that persisted despite adequate treatment for possible environmental exposures. Further specialized testing based on his initial workup led to his diagnosis. This case highlights the importance of sequential and targeted testing to evaluate for rare causes of eosinophilia.