In a patient with eosinophilic leukaemia, serial chromosome studies using the Giemsa banding technique revealed a similar marker chromosome, identified as an isochromosome 17, in all bone marrow metaphases analysed. There was no Ph-1-chromosome. The cytogenetic data support the view that eosinophilic leukaemia is a specific disease entity, and suggest that both eosinophilic and neutrophilic cells are involved in the leukaemic process.