Eosinophilia sometimes occurs in acute myeloid leukaemia (AML) and several laboratories have reported that cells with the t(8;21) or structural abnormalities of chromosome 16 can proliferate and differentiate to eosinophils in the presence of IL-5 in vitro. However, cases without these chromosomal abnormalities can also have eosinophilia. We investigated the association between the expression of IL-5 receptor (IL-5R) gene, karyotype and phenotype in 35 patients with AML. All cases expressed the IL-3R but the IL-5R gene was expressed predominantly in leukaemic cells with either t(8;21) or CD4-positive immunophenotype and was associated with the presence of eosinophilia.