BACKGROUND: The first sacral nerve root block (S1NRB) is a common procedure in pain clinic for patients complaining of low back pain with radiating pain. It can be performed in the office based setting without C-arm. The previously suggested method of locating the needle entry point begins with identifying the posterior superior iliac spine (PSIS). Then a line is drawn between two points, one of which is 1.5 cm medial to the PSIS, and the other of which is 1.5 cm lateral and cephalad to the ipsilateral cornu. After that, one point on the line, which is 1.5 cm cephalad to the level of the PSIS, is considered as the needle entry point. The purpose of this study was to analyze the location of needle entry point and palpated PSIS in S1NRB. METHODS: Fifty patients undergoing C-arm guided S1NRB in the prone position were examined. The surface anatomical relationships between the palpated PSIS and the needle entry point were assessed. RESULTS: The analysis revealed that the transverse and vertical distance between the needle entry point and PSIS were 28.7 +/- 8.8 mm medially and 3.5 +/- 14.0 mm caudally, respectively. The transverse distance was 27.8 +/- 8.3 mm medially for male and 29.5 +/- 9.3 mm medially for female. The vertical distance was 1.0 +/- 14.1 mm cranially for male and 8.1 +/- 12.7 mm caudally for female. CONCLUSIONS: The needle entry point in S1NRB is located on the same line or in the caudal direction from the PSIS in a considerable number of cases. Therefore previous recommended methods cannot be applied to many cases.