The hypereosinophilic syndromes (HES) are characterized by prolonged nonreactive peripheral blood hypereosinophilia with tissue damage. The lymphocytic HES variant can precede malignant clonal T-cell disease in adults but it is extremely rare to be the presenting feature of lymphomas in children. Here we present a 2.5-year-old boy with HES and mediastinal T-cell anaplastic lymphoma kinase (ALK) negative systemic anaplastic large-cell lymphoma. Mature and immature eosinophils without blasts were shown on bone marrow aspiration while biopsy revealed malignant infiltration. The patient responded well to initial corticosteroid therapy, but high-risk features make a challenge of finding the cure in this extremely rare case.