A child with eosinophilia, Loeffler endocarditis, and acute lymphoblastic leukemia.

We present an 8-year-old male with Loeffler endocarditis and acute lymphoblastic leukemia with hypereosinophilia (ALL/Eo) who initially presented with a 3-month history of peripheral eosinophilia thought to be due to visceral larval migrans. Despite treatment for Toxocara, his leukocytosis persisted and he developed mitral valve insufficiency and congestive heart failure. Myocardial biopsy revealed fibrosis and thrombus formation indicative of Loeffler endocarditis, and a peripheral smear showed pre-B-cell acute lymphoid leukemia. This unique case highlights a rare, yet serious sequella of prolonged eosinophilia.