Studies of X chromosome inactivation patterns are central to many aspects of our understanding of the pathogenesis of haematological malignancies. In patients with myeloproliferative disorders and myelodysplastic syndromes the demonstration of skewed X inactivation patterns in multiple haemopoietic lineages has been taken to indicate a stem cell origin for these groups of diseases. However, stem cell depletion or selection pressures can also produce skewed X inactivation patterns and might increase with age. We have therefore used the HUMARA assay to study X inactivation patterns of elderly patients with myeloproliferative disorders together with an age-matched control group of normal elderly women. A clonal pattern (clonal granulocytes and polyclonal T cells) was observed in 23.1% of normal women and 63.4% of patients with myeloproliferative disorders. This is the first report of X inactivation patterns in purified subpopulations of blood cells in normal elderly women. These results have three significant implications. Firstly, the finding of clonal granulocytes and polyclonal T cells in normal elderly women is likely to reflect age-related stem cell depletion or selection pressures. Secondly, the demonstration of clonal granulocytes and polyclonal T cells is not a useful diagnostic marker for myeloproliferative disorders or myelodysplastic syndromes in elderly women. Thirdly, our data raise the possibility that clonal blood cell patterns may precede rather than follow mutations which subsequently give rise to myelodysplastic or myeloproliferative phenotypes.