A 42-year-old woman presented with clinical and haematological features of essential thrombocythaemia (ET). Cytogenetic investigation revealed a standard t(9;22) Philadelphia translocation in all evaluable metaphases which persisted throughout treatment. Gene probe analysis of the chromosome 22 breakpoint cluster region (bcr) locus revealed a breakpoint mapping between exons 1 and 3 of the bcr gene, consistent with a standard bcr-abl translocation as found in chronic myeloid leukaemia (CML). Moreover, in separate cell populations, the bcr breakpoint was demonstrable in DNA from granulocytes but not in T cells from either peripheral blood or bone marrow. Since ET is known to be a clonal disorder with a multipotential stem cell origin, these findings suggest that, as in CML, the bcr-abl hybrid gene arises through translocation in a multilineage stem cell which does not involve the T lymphocyte lineage.