Skin biopsy specimens from 12 patients with plasmo-proliferative disease (myeloma and macrogobulinemia) were histopathologically and histochemically examined. In all the cases studied (by a biopsy obtained from the hand) there were alterations in the dermal microvasculature characterized by thickening of the PAS-stained basal membrane, endothelial swelling and proliferation with partial obliteration of the vascular lumen. The lesions were morphologically similar to those found in diabetic and paralymphomatous microangiopathy but in the present investigations greater endothelial proliferation has been observed. We did not find any lesions in the healthy control group. Different immunological, metabolic and hyperviscosity mechanisms are discussed. It is interesting to report that the results for blood coagulation and carbohydrate metabolism were normal; hyperviscosity was found in the serum of the patients examined, and in 3 cases cryoglobulinemia was also found. It will be interesting to carry out further investigations on the different pathogenic and biological aspects of the skin microvascular lesions in plasmo-proliferative diseases in order to increase our knowledge of the microcirculation in immunologic malignant disorders.