From 1950 through 1955, a total of 207 patients were seen at the Mayo Clinic with a diagnosis of polycythemia vera; 29 developed postpolycythemia myeloid metaplasia (PPMM). The patients with PPMM tended to have more symptoms than patients with agnogenic myeloid metaplasia. In contrast to agnogenic myeloid metaplasia, these patients had more pressure symptoms and bleeding symptoms, a more lethal syndrome, and a shorter survival. Of 130 patients treated with sodium phosphate P 32 for polycythemia vera, 25 developed PPMM. No retrospective hematologic or clinical factors could be uncovered to predict subsequent development of metaplasia. Ten of the initial 207 patients developed acute leukemia. Five of these ten, all men, went through a metaplastic phase prior to the onset of their leukemic process. Males with polycythemia rubra vera who go through a metaplastic phase may be at higher risk for the subsequent development of acute leukemia.