Posterior interosseous nerve palsy (PINP) is a rare complication of rheumatoid arthritis of the elbow. A 58-year-old woman with rheumatoid arthritis, who complained of an inability to extend her left fingers, was referred to our hospital. After a series of studies, extensor tendon ruptures were excluded, and PINP was diagnosed. By means of the Henry anterolateral approach, the radial nerve was exposed, which was compressed by swollen synovial membrane at the Frohse arcade. Elbow synovectomy was performed, and the arcade was opened to release the nerve. The PINP has been recovered completely within 2 weeks after surgery. Various examinations other than magnetic resonance imaging have been reported for the adjunctive diagnosis of PINP, but MR imaging was most useful as an adjunctive examination in this case.