Although some studies have shown that overexpression of vascular endothelial growth factor (VEGF) mRNA or protein is correlated with the progression of human malignancies, it is still unknown whether the VEGF level in tumor tissue correlates with tumor growth or metastasis. The present clinical study enrolled 26 patients with colon cancer and revealed that the VEGF level in tumor tissue was significantly higher than in adjacent normal tissue (220.93 +/- 217.64 pg/mg protein in the tumor tissue; n = 26; 38.93 +/- 20.26 in the normal tissue; n = 26) and significantly correlated with tumor size, whereas it did not correlate with other clinicopathological variables. The animal study involved orthotopic transplantation of a human colon cancer strain into nude mice and demonstrated that the VEGF level of transplanted tumor tissue (2,318.5 +/- 1,340.9 pg/mg protein) was significantly correlated with tumor weight (1,856.4 +/- 928.9 mg), but not with the number of the liver metastatic foci. These results indicate that VEGF produced by primary tumors of colon cancers may mainly promote primary tumor growth.