We report the case of a false aneurysm at the origin of the anterior tibial artery complicating upper tibial osteotomy. The proximally located lesion compressed the posterior tibial nerve, and despite successful decompression, the patient suffers from probably irreversible neurologic after-effects. Even though it is rare, this complication must be considered when faced with leg pain consecutive to upper tibial osteotomy without deep venous thrombosis.