Posterior interosseous nerve entrapment is a potential cause of upper extremity muscle weakness and pain. The diagnosis may be difficult to make clinically, and electrodiagnostic tests may not identify the exact site of nerve compression. We report a case of posterior interosseous nerve entrapment in which electrodiagnostic studies suggested radial neuropathy at the level of the spiral groove, but the sonographic evaluation pinpointed the arcade of Frohse as the level of compression. The patient's symptoms improved dramatically following surgical release of the nerve. Sonography may be a valuable, non-invasive diagnostic tool in evaluating patients with posterior interosseous nerve entrapment.