Neutropenia is the major factor predisposing to sepsis in cancer patients, and its duration is important for survival. We administered leukocyte transfusions (LT) to 10 children suffering from documented life-threatening infections during profound, prolonged neutropenia. Nine had acute leukemia, and one had aplastic anemia; three were bone marrow transplant recipients. These 10 were the only patients in our unit who received LT during the past 7 years. The median leukocyte dose was 0.6x10^9/kg in total. Instead of leukapheresis products, we used pooled buffy coats from random donors, with a high relative content of lymphocytes and especially T lymphocytes. The leukocyte concentrates were irradiated with 15 Gy. In all 10 patients, we observed prompt and sustained myeloid marrow recovery following LT. Such an effect of LT has never been described before. We hypothesize that in the internal milieu of these aplastic patients the transfused leukocytes were stimulated to secrete cytokines, the result being a cascade-like phenomenon and stimulation and proliferation of the patient’s own bone marrow cells. The bone marrow-stimulating effect of LT merits further study.