A 57-year-old man with a history of severe degenerative lumbar spine disease presented with painful proximal weakness of the right leg. Clinical examination suggested a femoral and obturator neuropathy with a palpable mass in the right groin. Magnetic resonance (MR) imaging disclosed a large synovial cyst of the underlying hip joint in the extrapelvic part of the iliopsoas and external obturator muscles, with femoral and obturator nerve compression. This case highlights the importance of detailed clinical examination in patients with multiple joint disease, the need for considering space-occupying cysts of degenerated joints as a potential cause of nerve damage in unusual locations, and the value of multiplanar MR imaging for proper diagnosis in such situations. Muscle Nerve, 2005.