Mouth opening limitation is generally caused by masticatory muscle or temporomandibular joint pain, disc dislocation without reduction, adhesion or ankylosis of the temporomandibular joint, and muscle contracture. But otorhinolaryngologic disease, neurologic and vascular disease, tumor, inflammation and infection may cause pain and mouth opening limitation which mimics temporomandibular disorders. Re-evaluation for possibilities of inflammation, infection and tumor should be in cases that do not show symptom improvement or appear with continuous aggravation despite of proper treatment. In this case, we report of medial pterygoid muscle pain and mouth opening limitation caused by periapical abscess of third molar spread to the pterygomandibular space.