Hepatocyte growth factor activator inhibitor type 1 (HAI-1) is an integral membrane Kunitz-type serine proteinase inhibitor initially identified as a potent inhibitor of hepatocyte growth factor activator (HGFA). HGFA is a serum proteinase that is critically involved in the activation of hepatocyte growth factor/scatter factor (HGF/SF) in injured tissue. Previous studies have shown that HAI-1 is expressed on the basolateral surface of various epithelial cells. In this study, we analyzed the expression of HAI-1 in human endothelial cells. Immunohistochemically, HAI-1 protein was observed in the endothelial cells of capillaries, venules and lymph vessels. On the other hand, arterial endothelial cells were poorly stained for HAI-1. Mesothelial cells on the serous surface were also positively immunostained. The endothelial expression of HAI-1 was also examined in cultured human endothelial cells of various origins, such as umbilical vein, microvessels and aorta. Notably, in accordance with the results of immunohistochemistry, HAI-1 mRNA and protein levels were high in the endothelial cells derived from umbilical vein and were hardly detectable in those derived from aorta. A low but distinct level of HAI-1 expression was also observed in endothelial cells from microvessels. As these HAI-1-positive endothelial cells also expressed MET tyrosine kinase, the specific receptor of HGF/SF, it is conceivable that HAI-1 might have an important regulatory role in the HGF/SF-MET signaling axis of endothelial cells, which could be involved in the process of angiogenesis.