Leukemic peripheral blood involvement in anaplastic large cell lymphoma (ALCL) is uncommon. We describe 3 children with such manifestations and review the features of 9 pediatric and adult patients previously described in the literature. Leukemic involvement in ALCL may occur at the time of initial diagnosis or develop during the course of disease. It most often is associated with the small cell histologic features and the t(2;5)(p23;q35). Clinical features commonly include significant respiratory distress, diffuse lung infiltrates or pleural effusions, and hepatosplenomegaly. Most cases have an aberrant T-cell immunophenotype with frequent expression of myeloid antigens, most often CD11b or CD13. Ten of the 12 cases reviewed had a poor response to therapy or early relapse. Thus, while anaplastic lymphoma kinase-positive ALCL and young patient age generally are associated with a favorable prognosis, leukemic involvement seems to identify a high-risk malignant neoplasm that requires more aggressive therapy, including hematopoietic stem cell transplantation.