The pronator quadratus is the main muscle responsible for pronation of forearm, helped by the pronator teres. To study the innervation of the pronator quadratus, eighteen forearms from a formol fixed corpses were dissected. We examined the relationship between the anterior interosseous nerve and the pronator quadratus. The wrist articular line was used as reference point. The branch which had the most proximal penetration into the pronator quadratus was 5.4 cm above the wrist articular line in a right forearm and 5.6 cm in a left one, and the branch which had the most distal penetration was 2.5 cm above the wrist articular line in a right forearm and 2.4 cm in a left one. The length of the anterior interosseous nerve and the width of the pronator quadratus muscle were measured. A great knowledge of the anatomical distribution of nerves could be of great help in treatment of the anterior interosseous syndrome