Purpose: The posterior interosseus nerve (PIN) can be difficult to locate within the radial tunnel. The nerve enters the supinator muscle passing under the arcade of Frohse, which is the proximal edge of the superficial part of the supinator muscle. The PIN courses through the superficial portion of the supinator muscle to exit distally. Anatomic landmarks could facilitate diagnosis and treatment of radial tunnel syndrome for injection and decompression of the PIN. Methods: 18 cadaver arms were used to identify anatomic landmarks to facilitate location of the PIN. The landmarks which were used included the palpable proximal radial edge of the radial head proximally and the mid-width of the wrist distally. Then the skin was incised down this longitudinal line through the fascia. Deep within this plane the PIN was identified exiting the distal edge of the superficial portion of the supinator muscle. This distal edge of the superficial supinator muscle was measured from the proximal radial aspect of the radial head. The course of the PIN was appreciated proximal and distal to the superficial part of the supinator muscle. Results: The PIN was identified to exit the superficial part of the supinator muscle an average distance of 7.4 +/- 0.4cm distal to the proximal radial aspect of the radial head. Distal to the distal edge of the supinator muscle, the PIN passed along the longitudinal vector of the radial head to the mid-width point of the wrist. Within the supinator muscle the PIN coursed in an oblique direction towards the lateral edge of the biceps tendon. Conclusion: The anatomic landmarks of the radial head and mid-width dorsal wrist can be used to predict the course and location of the PIN. The PIN can be predicted to exit the superficial part of the supinator muscle approximately 7.5cm distal to the radial head.