In 10 children with acute lymphoblastic leukaemia (mean leucocyte count 452 x 10(9)/l) 17 exchange transfusions were done for rapid reduction of this count. The control group comprised 11 children with similar values of white blood cell counts (mean 324 x 10(9)/l) treated initially with small, increasing doses of corticosteroids. In contrast to acute myeloid leukaemia the results do not support the usefulness of exchange transfusions in children with hyperleukocytosis during ALL.