A 45-year-old man with shortness of breath and eosinophilia.

A 45-year-old man who presented with dyspnea and chest tightness was found to have obstructive lung disease and eosinophilia of 10,300 eosinophils/μL. The differential diagnosis encompassed causes of primary eosinophilia and secondary eosinophilia associated with pulmonary disease, including asthma, environmental allergic reaction, eosinophilic granulomatosis with polyangiitis, allergic bronchopulmonary aspergillosis, acute eosinophilic pneumonia, chronic eosinophilic pneumonia, parasitic infections, tuberculosis, fungal infection, sarcoidosis, mastocytosis, drug reaction with eosinophilia and systemic symptoms, lymphoproliferative hypereosinophilic syndrome, and myeloproliferative hypereosinophilic syndrome. Infectious workup, fiberoptic bronchoscopy with biopsy, and tests for myeloproliferative mutations help differentiate among these causes. Identifying the underlying cause of eosinophilia is imperative in guiding treatment.