Background: Radial nerve (RN) compression most commonly occurs at the level of the supinator arch (SA), also called arcade of Fröhse, but other sites of entrapment along the course of the nerve are possible. This study aimed to perform an ultrasound and anatomical examination of these entrapment sites, to provide a solid anatomical base for the differential diagnosis of lateral arm pain, to allow a more precise manual therapy approach. Methods: Nineteen fresh-frozen cadavers were examined, first on ultrasound then on anatomical dissection. Two points of possible RN entrapment were injected with dye under ultrasound guidance: where the RN crosses the lateral intermuscular septum (LIMS) and at the SA. Dissection confirmed the location of the dye at these points and allowed us to describe the relationship of the RN with the adjacent structures; the distances from each of these two points to the lateral epicondyle and the diameter of the RN were also measured. Results: The dye was observed in the correct place in all specimens. We observed a close relationship of the RN with the lateral head of triceps brachialis (LHTB) muscle and the LIMS as it passed through these structures. In both structures, longitudinal aponeurotic extensions were observed. In the anterior compartment of the arm, where the RN glides between the brachialis (B) and brachioradialis (BR) muscles, we observed varying relationships between these three structures (5% had vascular unions, 79% had union of the epimysium, and 16% muscular unions). Finally, in the forearm, just before reaching the SA, we observed a septum that compartmentalize the forearm musculature and created an aponeurotic arch through which the motor branch of the RN passed. Conclusions: Ultrasound study helps correctly identify the RN; the two points identified on US and dissection correlated well. The anatomical findings on the relationship of the RN with its surrounding structures may explain its entrapment.