A fatal subarachnoid hemorrhage occurred in a patient receiving Coumadin therapy. An arteriogram 11 hours before her death failed to show an aneurysm. At autopsy, however, serial microscopic sections through the region of hemorrhage showed a break in a small pontine artery at the site of a muscularis deficiency. The hemorrhage was attributed to the giving way of the artery in an area of developmental weakness, while the fatal outcome probably was determined by the anticoagulation. This is the first time such a lesion has been recognized, and some heretofore-unexplained hemorrhages may have a similar explanation. © 1974 American Academy of Neurology.