BACKGROUND The purpose of this study was to describe detailed sonographic anatomy of the parasacral area for rapid and successful identification of the sciatic nerve. METHODS Fifty patients scheduled for knee surgery were included in this observational study. An ultrasound-guided parasacral sciatic nerve block was performed in all patients. The ultrasound probe was placed on an axial plane 8 cm lateral to the uppermost point of the gluteal cleft. Usually, at this level the posterior border of the ischium (PBI), a characteristically curved hyperechoic line, could be identified. The sciatic nerve appeared as a hyperechoic structure just medial to the PBI. The nerve lies deep to the piriformis muscle lateral to the inferior gluteal vessels, and if followed caudally, it rests directly on the back of the ischium. After confirmation with electrical stimulation, a 20-mL mixture of 1% ropivacaine and 1% lidocaine with epinephrine was injected. RESULTS The sciatic nerve was identified successfully in 48 patients (96%). In those patients, the median time required for its ultrasonographic identification was ten seconds [interquartile range, 8-13.7 sec], and the block success rate was 100%. CONCLUSION The described sonographic details of the parasacral area allowed for rapid and successful identification of the sciatic nerve.