The precise vascular territory and the variations of the acromio-thoracic axis were investigated in a series of 60 fresh cadavers and 50 formalin fixed specimens using dissection, ink injection and barium radiographic studies. The sternocostal portion and the clavicular head of the pectoralis major were found to have virtually independent vascular and nerve supplies. The pectoral artery supplied the former, whereas the deltoid artery nourished the latter. The dominant supply from the pectoral artery to the rib cage was found to enter around the fourth rib in the mid clavicular line. This supply is associated with a previously undescribed origin of the pectoralis major muscle in this region. The supply to the sternum was determined as indirect via the captured territory of the internal mammary system. The dominant supply to the skin from the pectoral artery arose laterally along the free lower border of the muscle as fasciocutaneous branches. The deltoid artery supplies the skin over the shoulder by numerous small branches which emerge from the intramuscular septa of the deltoid muscle. In addition a large axial artery was noted. In most cases this arose from the deltoid artery or its acromial branch and coursed laterally. It is noteworthy that the majority of skin paddles of the pectoralis major myocutaneous flap currently used in clinical practice are designed medially and inferiorly around the perimeter of the muscle and onto the rectus sheath. In these situations such flaps are not supplied directly by the pectoral artery. In fact, they are supplied indirectly by cutaneous branches belonging to the internal mammary/superior epigastric system which are captured by arterial connections with the pectoral artery. These occur predominantly in the pectoralis major muscle. Suggestions, based on these anatomical studies, are offered to improve the versatility and safety of flaps designed in this area.