Spontaneous resolution of hypereosinophilic syndrome in an infant without treatment.

We report on a 4-month-old boy with hypereosinophilic syndrome (HES) and spontaneous progressive resolution without treatment. Differential diagnosis excluded myeloproliferative, lymphocytic, familiar, associated, and overlap HES. The final diagnosis was undefined HES. Repeated measurements of blood eosinophil counts, monitoring of clonal T cells, and observation of skin lesions and organ involvement were carefully performed as an outpatient.