To determine whether human growth hormone (HGH) can cause proliferation of human leukemic lymphoblasts, we studied colony formation in semi-solid medium of MOLT 4, a cell line derived from an adolescent with acute lymphoblastic leukemia (ALL). Although exposure to single doses of HGH in supraphysiologic concentrations resulted in almost two-fold increases in number of colonies compared with control samples, physiologic concentrations had no effect. Similarly, physiologic concentrations of HGH had no effect on thymidine incorporation in short-term cultures of fresh lymphoblasts from children with ALL. In addition, total white blood cell and differential counts in 14 children with isolated growth hormone deficiency were reviewed pre- and post-treatment with HGH. In no case was there evidence of in vivo lymphocytosis or blastogenesis.