The number of survivors of childhood leukemia treated with GH for growth failure is increasing. The debate around the direct or indirect relationship of GH and IGF-I to the occurence of malignancy, especially in the case of GH therapy in patients with leukemia, is still unresolved. After we have studied the effect of GH and IGF-I on bone marrow (BM) of patients with acute leukemia (ALL and AML) in active disease, we studied patients with chronic myelogenous leukemia (CML) in remission. We have shown that GH increases blast colony count (BCC) by a mean of 68% and 77% at a GH concentrations of 250 and 300 ng/ml respectively. IGF-I increased BCC in all patients by 50, 93 and 105% at IGF-I concentrations of 0.05, 0.25 and 0.5 ng/ml respectively and in AML patients by 33, 58 and 65% in the presence of same concentrations. In colony forming assay did not reveal stimulation of peripheral blood blast colony forming by GH or IGF-I. In contrats with our in vitro data (as previously reported by us) that suggest that GH and IGF-I may promote blast cell proliferation, this effect was not on cells obtained from leukemia patients in remission.