Growth hormone (G.H.) or a G.H.-dependent somatomedin may be involved in the process of acute lymphoblastic leukaemia (A.L.L.). Growth hormone has a trophic effect on lymphoid tissue and also specific receptors on lymphocytes, most probably T cells. Hypophycess. Resting concentrations of G.H. and somatomedin activity are raised in some children with A.L.L. and may be reduced after remission is achieved. It is suggested that control of G.H. and/or somatomedin concentrations may be necessary for adequate treatment of some cases of A.L.L. in children.