An 11-year-old boy with mental retardation, malformations, and the mosaic karyotype 47,XY,+i(8p)/46,XY presented with fever, headache and petechiae. Peripheral blood WBC was 190 x 10(9)/l; and contained > 90% mature eosinophils. Cytogenetic analysis of the eosinophils revealed no aberrations except the constitutional karyotype. The patient was diagnosed as having a hypereosinophilic syndrome. Shortly after initiation of therapy he died from extensive mural thrombi of the heart and thrombi of several other organs. This is the first case of congenital triplication of the short arm of chromosome 8 associated with hypereosinophilic syndrome, suggesting involvement of genes on chromosome 8p in the regulation of eosinopoiesis.