Peroneus longus intra-tendinous ganglion cyst.

Ganglion cysts arising from tendons are uncommon lesions with an unknown cause. We present a case report of a 38-year-old female was diagnosed with an intra-tendinous ganglion cyst of the peroneus longus. She complained of right ankle swelling for 1 year and associated with pain. MRI revealed a peroneus longus intra-tendinous ganglion below the lateral malleolus with a thin wall. The ganglion cyst was surgically excised while the structure of the peroneus longus tendon was preserved. The clinical and functional outcomes were satisfactory after 1 year without recurrence.