The diameter of retinal arterioles, venules, and their macular branches was measured before and after macular laser photocoagulation in one eye each of six men and eight women with diabetic macular edema. The macular arteriolar branches constricted 20.2% (P < .001) and the venular branches constricted 13.8% (P < .001). This autoregulatory vasoconstriction results from the improved retinal oxygenation caused by the laser treatment. By extrapolating the principles of tissue edema formation in general, we hypothesized how macular laser treatment affects diabetic macular edema. Starling's law predicts that (laser-induced) vasoconstriction and reduced intravascular hydrostatic pressure should reduce edema formation in any tissue, including the retina.