The safety and efficacy of harvesting peripheral blood hematopoietic stem cells (PBSC) were evaluated in 38 children weighing 20 kg or less, with the smallest patient weighing 7 kg. The patients had a median age of 42 months and included 26 children with acute leukemias or lymphoma and 12 with various solid tumors. A total of 81 aphereses were performed, mostly in the recovery phase of chemotherapy, with or without granulocyte colony-stimulating factor, using a CS-3000 cell separator and regular procedure no. 3. Blood was withdrawn at a mean rate of 30 mL/min (range, 17 to 46 mL/min) through a temporary radial arterial catheter (20 to 24 guage) and returned through a larger catheter in a peripheral vein. Morbidity related to PBSC harvest was low and all aphereses were completed within 3 hours. The volume of blood per kilogram processed for each apheresis ranged from 85 to 615 mL (median, 270 mL). The median number of colony-forming units--granulocyte-macrophage (CFU-GM) and CD34+ cells collected were, respectively, 34 x 10(4)/kg and 15 x 10(6)/kg per apheresis and 126 x 10(4)/kg and 31 x 10(6)/kg per patient. Thirty-three patients (87%) required only a single apheresis to collect the minimum requirement of 10 x 10(4) CFU-GM/kg, including 28 patients (74%) from whom 30 x 10(4) CFU-GM/kg was obtained in a single apheresis. Twenty-three of the patients subsequently underwent autografts with PBSC. The median number of days required to achieve an absolute granulocyte count of 0.5 x 10(9)/L and a platelet count of 50 x 10(9)/L were, respectively, 10 (range, 6 to 15) and 14 (range, 9 to 46). The patients remained dependent on platelet transfusion support for a median of 10 days (range, 5 to 35). Thus, harvesting PBSC in very small children with active cancers is effective and safe and does not involve the risk of anesthesia or multiple invasive marrow aspirations.