Twenty-five patients with advanced essential thrombocythemia (ET; n = 13) or polycythemia vera (PV; n = 12) received hemopoietic stem cell transplants (HSCT) at the Fred Hutchinson Cancer Research Center. In most cases the indication to perform an HSCT was myelofibrosis with splenomegaly and peripheral blood cytopenias or the development of a myelodysplastic syndrome (MDS) or acute myeloid leukemia (AML). Patients were 18-60 (median 43) years old with intervals from diagnosis to HSCT of 8-348 (median 168) months. All but five patients had been treated with cytotoxic agents, and nine patients were splenectomized before transplant. Conditioning was performed with chemotherapy only or chemotherapy plus total body irradiation regimens followed by the infusion of either marrow (n = 19) or peripheral blood stem cells (n = 6) from related (n = 16) or unrelated (n = 9) donors. All evaluable patients showed sustained neutrophil engraftment. Nine patients (seven with AML/MDS, two with myelofibrosis) died of transplant-related complications, and 16 are surviving, 14 of them in continuous unmaintained remission. With a median follow-up of 41 (range 5-116) months after transplant, survival at 3 years is 64%. These data provide evidence that HSCT can be a curative treatment for patients with advanced PV and ET.