On attendance at the neurovascular clinic at the Manchester Royal Infirmary, patients with the symptoms of intermittent claudication undergo a routine clinical examination which enables the underlying vascular disease to be assessed (Boyd et al., 1949). In several cases the examination failed to reveal any underlying pathological condition. Subsequent investigation showed that in two cases the lesion was a unilateral thrombosis of the external iliac artery. As a search of the literature has not produced any description of such a lesion and its sequelae, a detailed account of these two cases is given. No discussion is made of other and now well-recognized causes of "proximal" arterial block such as the aortic bifurcation syndrome (Leriche and Morel, 1948).