Hydroxyurea was administered orally to prevent the effects of leukostasis in adults with acute leukemia who had peripheral blast cell counts greater than 100,000/cu mm. A single oral dose of 50 to 100 mg/kg was given daily until the absolute blast cell count decreased to less than 100,000/cu mm. Hydroxyurea was effective in rapidly lowering the blast cell count by an average of 50% after one dose in each of ten episodes. No patient developed symptoms or signs of the leukostasis syndrome, and no side effects directly attributable to hydroxyurea were observed. The leukostasis syndrome associated with very high blast cell counts in adults with acute leukemia can be avoided by the use of hydroxyurea in the manner described. This treatment can be particularly useful in the interval before consultation or referral and prior to the cytotoxic effect of definitive induction chemotherapy.