Aetiological investigation should rule out myeloid and lymphoid neoplasms, as eosinophil growth and accumulation can either be due to an intrinsic defect of eosinophil-committed neoplastic progenitor cells, caused by mutations including platelet-derived growth factor receptor (PDGFR) or fibroblast growth factor receptor 1 (FGFR1), or cytokine overproduction, such as interleukin-3 (IL-3) and IL-5, that stimulate the growth, differentiation, and survival of eosinophils. This evidence concerns the gene IL3 and lymphoid neoplasm.