Monosomy 7 is the commonest monosomy in acute myeloid leukaemia (AML). A patient with AML and monosomy 7 was investigated by fluorescence in situ hybridization. Monosomy 7 could be demonstrated in the blasts and also in mature eosinophils, but apparently not in lymphocytes. Monosomy 7 may occur in a multipotential myeloid progenitor cell as part of the multi-step leukaemogenic process.