Basement membranes (BMs) consisting of laminins, collagens, and heparan sulfate proteogly cans (HSPGs) are vital for proper endothelial cell function, but many aspects of their role in vascular development remain unknown. Here, we demonstrate that vascular structures within differentiating embryoid bodies are wrapped in a BM composed of α4‐ and α5‐chain laminins, fibronectin, collagen IV, and HSPGs. In sprouting angiogenesis, laminins were pro duced by stalk cells, as well as the leading tip cell, and deposited along the sprout length, including tip cell filopodia. In embryonic stem cells deficient in laminins, due to lamc1 (laminin γ1) deletion, vascular develop ment and organization were largely unaffected. How ever, the frequency of vessels with wide lumens was increased 4‐fold. Laminin‐deficient vessels were more over characterized by increased fibronectin levels and enhanced endothelial cell proliferation. We conclude that laminins are dispensable for vascular development but that they regulate lumen formation in the absence of flow and vascular tone.—Jakobsson, L., Domogatskaya, A., Tryggvason, K., Edgar, D., Claesson‐Welsh, L. Laminin deposition is dispensable for vascu‐ logenesis but regulates blood vessel diameter independent of flow. FASEBJ. 22, 1530–1539 (2008)