OBJECTIVE To quantify the variation in the blood supply of the retrolaminar optic nerve in humans. The retrolaminar anterior optic nerve is supplied by an elliptical arterial "circle" of Haller and Zinn formed by anastomoses around the optic nerve between medial and lateral paraoptic short posterior ciliary arteries (PO-SPCAs). The frequency with which complete perioptic nerve arteriolar anastomoses (PONAA) occur is unknown, yet they form the basis for many postulated pathophysiological mechanisms. METHODS Scanning electron microscopy was performed on 25 orbital and ocular microvascular corrosion casts (methyl methacrylate) from 20 human cadavers of subjects 21 to 97 years of age without known ocular disease. RESULTS Eighteen casts were examined in detail after microdissection (the analysis of seven casts was limited because of their fragility). In 15/18 (83%) casts, the PONAA were supplied by branches of a medial and lateral PO-SPCA; in 2/18, they were supplied by one (lateral) PO-SPCA; and in 1/18, they were supplied by a superior PO-SPCA and two horizontal PO-SPCAs. The PONAA had intact superior and inferior anastomoses in 8/18 (44%) casts, anastomoses with narrowed portions in 6/18 (33%), and incomplete in 4/18 (23%). Narrowed portions were not preferentially distributed to either superior or inferior anastomoses. CONCLUSIONS Complete PONAA was found in more than 75% of casts, including anastomoses with narrow portions in 33% of casts.