Despite the expansion of arthroscopic surgery of the shoulder, the open deltopectoral approach to the shoulder is still frequently used, for example in fracture fixation and shoulder replacement. However, it is sometimes accompanied by unexpected bleeding. The cephalic vein is the landmark for the deltopectoral interval, yet its intimate relationship with the deltoid artery, and the anatomical variations in that structure, have not previously been documented. In this study the vascular anatomy encountered during 100 consecutive elective deltopectoral approaches was recorded and the common variants described. Two common variants of the deltoid artery were encountered. In type I (71%) it crosses the interval and tunnels into the deltoid muscle without encountering the cephalic vein. However, in type II (21%) it crosses the interval, reaches the cephalic vein and then runs down, medial to and behind it, giving off several small arterial branches that return back across the interval to the pectoralis major. Several minor variations were also seen (8%). These variations in the deltoid artery have not previously been described and may lead to confusion and unexpected bleeding during this standard anterior surgical approach to the shoulder. Cite this article: Bone Joint J 2013;95-B:657–9