In a recent study we demonstrated that recombinant human growth hormone (r-hGH; Saizen) delayed tumor-induced cachexia in human tumor xenografts in vivo. Such a therapeutic effect could have a great impact in the supportive care of advanced cancer patients. Before large clinical studies are initiated possible growth stimulation should be excluded. This question was investigated in vitro in 20 human tumor models, which had been established in serial passage in nude mice. The effect of continuous exposure of r-hGH was investigated at dose levels ranging from 0.3 ng/ml up to 0.1 microg/ml in colorectal (n=2), gastric (n=1), non-small cell lung (n=4), small cell lung (n=1), mammary (n=3), ovarian (n=2), prostate (n=2) and renal cancers (n=2), and melanoma (n=3) using a modified Hamburger and Salmon clonogenic assay. The results show that there was neither tumor growth inhibition nor any evidence for tumor growth stimulation in any of the tumors studies. Therefore this preclinical study in 20 human tumor models indicated no direct risk for tumor growth enhancement.