Recently, new potent antagonistic analogs of growth hormone-releasing hormone (GH-RH) have been synthesized. These GH-RH antagonists bind to pituitary receptors for GH-RH and inhibit the release of GH in vitro and in vivo. This suggests that they could be clinically useful in conditions such as acromegaly. The main applications of GH-RH antagonists would be in the field of insulin-like growth factor I (IGF-I)- and IGF-II-dependent cancers. GH-RH antagonists inhibit the growth of various human cancer cell lines xenografted into nude mice, including mammary cancers, androgen-independent prostate cancers, small-cell lung carcinomas, non-small-cell lung carcinomas, renal adenocarcinomas, pancreatic cancers, colorectal carcinomas and malignant gliomas. These effects could, in part, be exerted indirectly through inhibition of the secretion of GH and the resulting reduction in levels of hepatic IGF-I. However, the principal action of GH-RH antagonists in vivo appears to be the direct suppression of the autocrine and/or paracrine production and expression of the genes encoding IGF-I (IGF1) and IGF-II (IGF2) in tumors. In vitro, antagonists of GH-RH inhibit the proliferation of mammary, prostatic, pancreatic and colorectal cancer cell lines, reducing the expression of IGF2 mRNA in the cells and the secretion of IGF-II. The presence of the GH-RH ligand has been demonstrated in human ovarian, endometrial, mammary and lung cancers, suggesting that GH-RH could be a growth factor. Further development of GH-RH antagonists should lead to potential therapeutic agents for IGF-dependent cancers.