Pancreatic cancer has a poor prognosis, and treatment strategies based on preclinical research have not succeeded in significantly extending patient survival. This failure likely stems from the general lack of information on pancreatic tumor physiology, attributable to the difficulties in developing relevant, orthotopic models that accurately reflect pancreatic cancer in the clinic. To overcome this limitation, we developed abdominal wall windows suitable for intravital microscopy that allowed us to monitor angiogenesis and microvascular function noninvasively during tumor growth in vivo. We used two complementary tumor models in mice: orthotopic (human ductal pancreatic adenocarcinoma, PANC-1, grown in the pancreas), and ectopic (PANC-1 grown in the abdominal wall). We found that orthotopic PANC-1 tumors grew faster than the ectopic tumors and exhibited metastatic spread in the late stage similar to advanced pancreatic cancer in the clinic. Orthotopic PANC-1 tumors expressed vascular endothelial growth factor (VEGF)(121) and VEGF(165), contained higher levels of tumor cell-derived VEGF protein, and maintained vascular density and hyperpermeability during exponential tumor growth. Orthotopic PANC-1 tumors showed lower leukocyte-endothelial interactions in the early stage of growth. In addition, both VEGF(121) and VEGF(165) promoted the growth of PANC-1 cells in vitro. Finally, Anti-VEGF neutralizing antibody inhibited angiogenesis and tumor growth of PANC-1 tumors in both sites. We conclude that the orthotopic pancreas microenvironment enhances VEGF expression, which stimulates growth of PANC-1 tumors (compared with ectopic tumors). The mechanism is autocrine and/or paracrine and also is involved in the maintenance of blood vessels. This comparative system of orthotopic and ectopic pancreatic cancer will provide the rigorous understanding of pancreatic tumor pathophysiology needed for development of novel therapeutic strategies.