The authors present a case of venous air embolism occurring immediately upon skin closure after craniotomy in the prone position. This 5-year-old patient had a third ventricle tumor resected with bipolar cautery via a frontal trans-collosal approach into the lateral ventricle and through the foramen of Monroe. Doppler monitoring was utilized during the case since the patient's head was extended upwards in 10° reverse Trendelenburg position. No air was detected during the operation. The ventricles were filled with saline presumably displacing air, prior to dural closure. However, with an increase in nitrous oxide from 55 to 68% prior to skin closure, venous air embolism was subsequently detected by Doppler and confirmed by end-tidal/arterial pCO2 gradient. The authors speculate that tension pneumocephalus caused the venous air embolism and describe the probable route of entry into the venous system.