Emergency providers are likely to encounter patients with acute and chronic leukemias. In some cases, the first presentation to the emergency department may be for symptoms related to blast crisis and leukostasis. Making a timely diagnosis and consulting a hematologist can be life saving. Presenting symptoms are caused by complications of bone marrow infiltration and hyperleukocytosis with white blood cell counts over 100,000. Presentations may include fatigue (anemia), bleeding (thrombocytopenia), shortness of breath, and/or neurologic symptoms owing to hyperleukocytosis and subsequent leukostasis. Treatment of symptomatic cases involves induction chemotherapy and/or leukapheresis. Asymptomatic hyperleukocytosis can be treated with hydroxyurea.