The distribution of proteoglycans in 21 temporal and two ulnar artery pseudoaneurysms was studied immunohistochemically. A history of trauma was elicited in six cases, and 16 of the lesions were pulsatile. The clinical diagnosis was aneurysm or cyst in 18 patients, possible arteritis in two patients, tumor in one patient, and unknown in the remaining patient. Histologically, there was a prominent myxoid neointimal response in the walls of each interrupted artery. The remnant arterial segment was often inconspicuous. Prominent smooth muscle cell proliferation and granulation tissue response with inflammation led to misdiagnosis of tumor or vasculitis, respectively, in 11 cases. Immunohistochemical staining for proteoglycans demonstrated abundant, diffuse versican in interrupted wall segments. Biglycan was confined to collagenized and vascularized areas. In some portions of medial disruption, in which angiogenesis was prominent, decorin was expressed within endothelial cells of neocapillaries. These findings demonstrate that superficial pseudoaneurysms may be mistaken clinically and pathologically for unrelated entities. The immunohistochemical studies confirm that versican is upregulated in areas of tensile stress. In addition, the presence of endothelial expression of decorin supports the concept of decorin's involvement in angiogenesis.