The existence of tumor cell-lined sinuses (vascular channels) in various experimental and human cancer is known for almost a half a century described by a Hungarian pathologist, Béla Kellner. Meanwhile, even the existence as well as the pathomechanism and the possible functional significance of these sinuses are heavily challenged in the recent literature. Ultrastructural studies however provide evidence for the presence of tumor cell-lined sinuses in human melanoma and breast cancer. The generation of such sinuses can be suggested in two ways: by de novoformation, when tumor cells recapitulate an embryonic geno- and phenotype by reexpressing endothelial genes or by a secondary mechanism, where the incorporated microvessels degenerate due to the predominant expression of anti-angiogenic factors. Literature data are available for the potential diagnostic and clinical significance of the tumor sinuses (vascular channels) stimulating further studies on this issue.