In a prospective, controlled trial 26 anaemic, neutropenic children with newly diagnosed acute lymphocytic leukaemia were randomised in pairs to receive either transfusion to a haemoglobin of 10--12 g/dl where clinically indicated (group A) or hypertransfusion to a haemoglobin of 16--18 g/dl (group B). Compared with group A (11 of 13 transfused), group B (all transfused) had a significantly more rapid rise in neutrophils at 7 and 10 days post-transfusion, a lower incidence of infection, and less interruption to chemotherapy. Hypertransfusion restored the myeloid/erythroid ratio to normal in bone-marrow of 5 of 6 children and the proportion of early myeloid precursors was greater than in controls.