Posterior interosseous nerve syndrome is an entrapment neuropathy of the deep terminal branch of the radial nerve -the posterior interosseous nerve -at the radial tunnel. Radial neuropathy without prior injury or other obvious etiology occurs frequently. However, sometimes previous trauma, surgery, or tumors over the radial nerve contribute to chronic entrapment and can produce the classic syndrome. In this report we describe an unusual case of a 68-year-old-woman with a chronic posterior interosseous nerve syndrome due to a lipoma. The patient had poor recovery after surgery, despite splinting and rehabilitation physiotherapy, mainly because of the long duration (2 years) and severity of the compression neuropathy.