The ultrastructural features associated with vascular permeability in 9 cases of advanced gastric carcinomas were studied, and compared with that of control non-neoplastic mucosa. Tumour microvasculature showed features in common with those of control mucosa, including complete basal lamina, well-developed interendothelial junctions, fenestrations and caveolae. Some tumour blood vessels showed endothelial cell swelling accompained by luminal narrowing and perivascular fibrosis. In 2 out of 9 cases, there were endothelial attenuation with numerous fenestrations and vesiculo-vacuolar organelles. The vesiculo-vacuolar organelle is a recently described cytoplasmic structure found in the endothelial cells lining tumour microvessels and normal venules and which provides an important pathway for extravasation of circulating macromolecules. Our ultrastructural data suggest that advanced gastric carcinomas share with experimental tumour models in vivo only some morphologic features associated with hyperpermeability including fenestration, endothelial attenuation and vesiculo-vacuolar organelles. The implications of perivascular fibrosis on the delivery of immune cells to gastric carcinomas are discussed.