© 2016 Surgical Neurology International | Published by Wolters Kluwer-Medknow.Background: To describe the rare fnding of a double massa intermedia (MI). Typically, the MI (interthalamic adhesion) is a single bridge of gray matter connecting the medial surfaces of the thalami. Methods: Twelve formalin-and alcohol-fixed human third ventricles were examined from superior to inferior by fiber dissection technique under ×6 to ×40 magnifcations and with the endoscope. Results: In all hemispheres, the anterior and posterior commissure were defned. The MI, which bridges the medial surfaces of the thalami, was defned in all hemispheres. In one hemisphere, there was a second bridge between the thalami, located posteroinferior to the common MI. Endoscopic view confrmed that there was a second MI in this specimen. The MI usually traverses the third ventricle posterior to the foramen of Monro and connects the paired thalami. The MI is an important landmark during endoscopic and microscopic surgeries of the third ventricle. Although a double MI is very rare, surgeons should be aware of the possibility in their surgical planning. Conclusion: The surgeon should be aware of the possibility of a double MI to avoid confusion during third ventricle surgery.