Effect of resistance to HU on survival and on risk of transformation to acute leukemia or myelofibrosis. (A) Effect of resistance to HU on survival. The survival curves were adjusted for age, sex, hematologic values at diagnosis, thrombosis, hemorrhage, and WBC count response. (B) Effect of resistance on the risk of transformation to acute leukemia or myelofibrosis. The cumulative incidence of transformation was computed taking death as a competing risk and adjusted for age, sex, hematologic values at diagnosis, thrombosis, hemorrhage, WBC count response, and exposure to busulfan or radioactive phosphorus.