Interleukin 1 (IL-1) is a pluripotent cytokine that promotes angiogenesis, tumor growth, and metastasis in experimental models; its presence in some human cancers is associated with aggressive tumor biology. The purpose of these studies was to characterize the role of IL-1 in human cancers and determine if inhibition of IL-1 via its receptor antagonist, IL-1Ra, alters tumor growth and metastatic potential. IL-1 mRNA or protein levels were determined in clinical tumor samples, cancer cell lines, and xenografts using quantitative reverse transcription-PCR or ELISA. Biological activity of tumor-derived IL-1 protein was shown via induction of permeability across endothelial cell monolayers. The effects of recombinant IL-1Ra on tumor lines in culture (cell proliferation and IL-8 secretion) and in xenograft models (tumor growth, metastatic potential, and intratumoral levels of IL-8 and VEGF) were characterized. The effects of IL-1Ra-mediated regression of xenograft growth on angiogenic proteins (IL-8 and VEGF) were evaluated in an IL-1-producing melanoma (SMEL) xenograft model. IL-1 mRNA was highly expressed in more than half of all tested metastatic human tumor specimens including non-small-cell lung carcinoma, colorectal adenocarcinoma, and melanoma tumor samples. Constitutive IL-1 mRNA expression was identified in several cancer cell lines; tumor supernatant from these cell lines produced a significant increase in endothelial cell monolayer permeability, a hallmark event in early angiogenesis, in an IL-1-dependent manner. Moreover, systemic recombinant IL-1Ra resulted in significant inhibition of xenograft growth and neovessel density of IL-1-producing, but not non-IL-1-producing, tumor cell lines. Subsequent analysis of SMEL, a melanoma cell line with constitutive IL-1 production, showed that neither exogenous IL-1 nor IL-1Ra altered tumor cell proliferation rates in vitro. Gene expression analyses of IL-1Ra-treated SMEL xenografts showed a >3-fold down-regulation of 100 genes compared with control including a marked down-regulation of IL-8 and VEGF. These data show that the IL-1 gene is frequently expressed in metastases from patients with several types of human cancers. IL-1Ra inhibits xenograft growth in IL-1-producing tumors but has no direct antiproliferative effects in vitro; decreased tumor levels of IL-8 and VEGF may be an early surrogate of IL-1Ra-mediated antitumor activity. IL-1Ra may have a role alone or with other agents in the treatment of human cancers.