The results of treatment by Seidel locked nailing in 13 recent fractures and 4 nonunions of the humerus are reported. The feature common to all of the fractures was instability consequent to comminution or in the site of the fracture center. The 4 nonunions were originally fractures synthesized by Rush intramedullary nailing. The surgical method used an entry approximately 1 cm from the greater tuberosity through a well-vascularized area of the rotator cuff. All of the cases achieved consolidation and in 70% of the patients results were excellent with complete recovery of range of movement of the shoulder. Thus, the Seidel nail appears to be an intramedullary means of synthesis particularly suited to the treatment of unstable fractures and nonunion of the humerus because it guarantees stability, especially in rotation, superior to any other intramedullary means of synthesis.