Growth hormone inhibits tumor metastasis. The effect of growth hormone on tumor growth and metabolism in the tumor-bearing host is unknown. This study was done to determine the effect of recombinant growth hormone on primary tumor growth, tumor metastasis, and carcass weight in tumor-bearing animals. Twenty-seven female Lobund/Wistar rats with subcutaneous prostate tumor implants (PA-III) were randomized to receive a standard protein diet (22.0% protein; 4.27 kcal/g) or an isocaloric, protein-depleted diet (0.03% protein; 4.27 kcal/g) ad libitum orally. One half of the animals in each group were randomized to receive daily injections of either recombinant growth hormone (1000 mU/kg/day intramuscularly) or placebo (saline) for 14 days. A significant increase in body weight was observed in growth hormone-treated animals without acceleration of primary tumor growth. Spontaneous pulmonary metastasis was inhibited significantly in animals in both dietary groups treated with growth hormone. Thus, growth hormone selectively supports host growth and inhibits pulmonary metastasis in this tumor-bearing animal model. The potential metabolic effects and clinical consequences of treating cancer patients with growth hormone is discussed. 