Clavicle fractures are common, with the majority treated conservatively. If treated conservatively, pseudarthrosis of the clavicle is reported in up to 3% of the cases. In rare cases, pseudarthrosis of the clavicle may cause pseudoaneurysm formation, resulting in compression of the brachial plexus and the adjoining vessels, which may produce neurological symptoms and circulatory disorders. Here, we describe two cases of the late onset of pseudoaneurysm formation after pseudarthrosis of the clavicle. Both cases were remarkable because they showed clinical symptoms of TOS. Therefore, surgical treatment was performed and included claviculectomy, resection of the pseudoaneurysm and interposition grafting with an artificial prosthesis. One year after the operation, both patients showed excellent upper extremity function without any deficit of vascular, sensorial or motorial function. Patient's history and radiological findings are the keys to diagnosis. Without treatment, the prognosis is poor with spontaneous development of bleeding or gangrene. Therefore, surgical treatment has to be performed, especially when neurological symptoms occur.