Leptomeningeal venous drainage of cranial dural arteriovenous fistulae is the most important determinant of adverse clinical course. Factors that predispose to its occurrence have not been adequately addressed in the literature. In the present study, we investigated the relation of shunt location to the development of leptomeningeal venous drainage, with regard to the bridging veins. Angiographic data of 211 consecutive patients with cranial dural arteriovenous fistulae treated over 19 years were analyzed. Dural shunts with leptomeningeal venous drainage were found in 107 patients; of these, 71 patients had pure leptomeningeal venous drainage (Borden type 3). The angioarchitecture of the shunt, including pattern of arterial feeders, relation with the bridging veins, primary venous drainage, and venous outflow restrictions were recorded. After analysis of the 71 Borden type 3 shunts with exclusive leptomeningeal venous drainage, three patterns emerged. The commonest was the fistula engaging a bridging vein that had lost its connection to the parent sinus into which it previously drained; it was characterized by an arterial network of feeders converging onto the wall of a bridging vein, with leptomeningeal venous reflux. The other patterns were those of "isolated" sinus segment characterized by arterial feeders converging on to the wall of the dural sinus with leptomeningeal venous reflux following the opacification of the sinus and fistulae in the vicinity of the cribriform plate with two subtypes. The main angioarchitectural features of the 36 Borden type 2 shunts with mixed sinusal-cortical venous drainage were the presence of a diffuse arterial network of vessels converging onto a site in the wall of the dural sinus, with leptomeningeal venous reflux following the opacification of the sinus. In this group, four exceptions were noticed with arterial feeders converging onto a bridging vein and having a mixed venous drainage to the cortical venous system and the sinuses. We concluded that the exact location of the shunt with regard to the bridging veins is a key factor in the development of leptomeningeal venous drainage. Cranial dural arteriovenous fistulae (CDAVFs) of either Borden type 2 or 3 do not constitute a homogeneous group. The great majority of these shunts present thrombotic phenomena.