A case with a large vertebrobasilar junction aneurysm developed a venous infarction in the temporal lobe after an operation using the transpetrosal approach. Although little of the literature has been concerned with venous complications after the transpetrosal approach, the case prompted us to study the venous system as playing a key role in the transpetrosal approach. Analyzing 30 carotid and 15 vertebral angiograms of 15 patients who underwent preoperative cerebral angiography using digital subtraction angiography (DSA), we investigated the venous system near the junction of the superior petrosal sinus, the transverse sinus and the sigmoid sinus (STS junction) which may play a key role in the transpetrosal approach. Drainage pathways of the superficial middle cerebral vein (SMCV) were classified into four types; sphenoparietal, sphenobasal, sphenopetrosal and undeveloped. In the sphenopetrosal type (4/30: 13%), the drainage of SMCV passes back along the floor of the middle fossa to drain into the transverse sinus. The lateral temporal vein (LTV) and the temporobasal vein (TBV) drain into the transverse sinus. The LTV emptied into the transverse sinus either directly (20/30: 67%) or indirectly through the tentorial sinus (10/30: 33%). The entry of the LTV into the transverse sinus (venous point) was usually located in the lateral third of the transverse sinus (14/20: 70%), but sometimes in the middle third (6/20: 30%). The TBV, observed in 8/30 (27%), also often emptied into the tentorial sinus to drain into the transverse sinus. Atresia of a unilateral transverse sinus and a large LTV emptying into the distal sigmoid sinus was observed in 2/15 cases. The venous system near the STS junction may be interrupted by the incision of the tentorium and the middle fossa dura mater and by the retraction of the sigmoid sinus. Since the transpetrosal approach may cause venous complication by compromising the venous system near the STS junction, it is necessary to evaluate of the venous system preoperatively using DSA and to set up a surgical strategy preserving the venous system.