Angiopoietin (Ang)-1 and -2 are critical regulators of embryonic and postnatal neovascularization. Ang-1 activates the endothelial cell-specific tyrosine kinase receptor Tie-2, which in turn leads to enhanced endothelial cell survival and stabilization. The effects of Ang-1 on tumor angiogenesis remain controversial; although we have previously demonstrated that Ang-1 overexpression in colon cancer cells leads to a decrease in s.c. tumor growth, others have shown that Ang-1 may be proangiogenic. Few studies have addressed the role of the Angs in tumors growing in the organ of metastatic growth. We hypothesized that overexpression of Ang-1 may inhibit the growth of colon cancers growing in the liver by inhibition of angiogenesis. We also wanted to investigate the mechanisms by which Ang-1 affects angiogenesis in vivo. Human colon cancer cells (HT29) were stably transfected with an Ang-1 construct or an empty vector (pcDNA) and injected directly into the livers of nude mice. After 37 days, livers were harvested and weighed, and tumor sizes were measured. In an additional experiment, to validate the paracrine effect of Ang-1, various mixtures of control cells and Ang-1-transfected cells were injected into livers, and tumor growth was assessed. Direct effects of recombinant Ang-1 on angiogenesis were studied with an in vivo Gelfoam angiogenesis assay. The impact of Ang-1 on vascular permeability was investigated using an intradermal Miles assay with conditioned media from transfected cells. Liver weights (P < 0.05), tumor volumes (P < 0.05), vessel counts (P < 0.01), and tumor cell proliferation (P < 0.01) in the Ang-1 group were significantly lower than those in the control (pcDNA) group. Tumor vessels in the Ang-1 group developed a significantly higher degree of pericyte coverage (P < 0.02) than vessels in pcDNA tumors. In the cell mixture experiment, even as few as a 1:10 mixture of Ang-1-transfected cells/control cells resulted in a significant reduction of hepatic tumor volumes (P < 0.04). In the angiogenesis assay, vessel counts in Gelfoam implants were significantly decreased by the addition of Ang-1 (P < 0.01). Finally, conditioned medium from Ang-1-transfected cells decreased vascular permeability more than that from control cells (P < 0.05). Our results suggest that Ang-1 is an important regulator of angiogenesis and vascular permeability and that this effect may be secondary to increasing periendothelial support and vessel stabilization. Thus, Ang-1 could potentially serve as an antineoplastic or anti-permeability agent for patients with metastatic colorectal cancer.