Lesions in the nerves supplying the flexor digitorum profundus cause a loss of flexibility in the distal phalange of the middle phalange of the finger. A knowledge of the distribution of the nerve branches, and of their origins and points of penetration in the forearm would make the treatment of these lesions easier, especially when surgery is involved. In this study, we dissected 20 forearms from 10 formol-fixed corpses and examined the relationships of the anterior interosseous and ulnar nerves to the flexor digitorum profundus. The articular lines of the elbow and wrist were used as reference points. In the left upper limb, most branches of the anterior interosseous nerve (39.3%) began in the distal half of the proximal third of the forearm, whereas most branches of the ulnar nerve (54.5%) began in the distal third of the arm. As a result, most of the anterior interosseous (52.7%) and ulnar (64.9%) nerve branches penetrated in the distal half of the proximal third of the forearm. In the right upper limb, most of the branches of the anterior interosseous nerve (35.7%) began in the distal half of the proximal third of the forearm, whereas the branches of the ulnar nerve (50%) began in the distal third of the arm and in the distal half of the proximal third of the forearm. Most of the penetration points for the anterior interosseous (45.8%) and ulnar (62.8%) nerve branches began in the distal half of the proximal third of the forearm. Anatomical variations were observed in some cases.