Patients with acute myelocytic leukemia and hyperleukocytosis have a poor prognosis due to vascular leukostasis and infiltration in the central nervous system and lungs. The clinical records of all patients with a new diagnosis of acute myelocytic leukemia and initial white blood cell count greater than 100,000 X 10(9)/liter admitted to the Mount Sinai Medical Center between the years 1974 and 1983 were examined. Forty-three patients were identified, 22 of whom had clinical and/or pathologic evidence of leukostasis of the central nervous system and/or lung. All patients received induction chemotherapy with daunorubicin and a continuous infusion of cytosine arabinoside. Thirty-five patients also underwent therapeutic leukapheresis prior to induction chemotherapy. The overall remission induction rate in these 43 patients was 51 percent. Fifteen patients had lung leukostasis; the remission rate for these patients was 27 percent (three complete remissions, one partial remission), as compared with a remission rate of 64 percent (18 of 28) for those without pulmonary leukostasis (chi 21 = 5.53; p = 0.02). Thirteen patients had central nervous system infiltration; the remission rate for these patients was 46 percent (five complete remissions, one partial remission), as compared with 53 percent (16 of 30) for patients without central nervous system involvement (chi 21 = 0.19; p = 0.67). The median survival of 21 patients without leukostasis was 10.8 months, as compared with 15.4 months for seven patients with central nervous system involvement and no lung leukostasis and 0.2 months for 15 patients with pulmonary leukostasis (chi 22 = 19.9; p less than 0.001). Thus, pulmonary leukostasis was found to be the single worst prognostic factor in this group of patients.