The prevailing concept of etiologic heterogeneity for the diabetes mellitus syndrome is one of multiple genetic factors interacting with a variety of environmental influences. Variation in expression of the disorder, particularly the need for insulin, does not correlate with known etiologic distinctions. There is much evidence for genetic heterogeneity, as well as phenotypic variation when etiology can be presumed to be identical. The vascular manifestations of diabetes include microangiopathy unique to diabetes and larger vessel disease that differs from that of normal aging only by its prematurity. There is as much evidence for heterogeneity of the vascular expression as there is for glucose intolerance. Approximately 25% of persons with insulin-dependent diabetes may never develop the microvascular disease. The pathogenesis of vascular disease in diabetes may involve a number of abnormalities of plasma, circulating cells, and vascular tissue. Were absolute control of glycemia possible, some of the contributing factors involved in vasculopathy would possibly be alleviated. In the absence of automated physiologic insulin replacement the potential deleterious effect of our current methods of treatment might be reduced by specific inhibition of excess catecholamine, growth hormone and/or glucagon responses.