In discussions of vascular complications of diabetes the fact that capillary basement membranes are, in general, thickened (CBMT) in poorly controlled diabetics is no longer at issue. However, three important questions concerning the pathophysiologic significance of CBMT remain unanswered: (1) How and why do capillary basement membranes thicken in diabetes? (2) What is the functional significance of capillary basement membrane changes in diabetes? (3) What is the nature of the relationship of CBMT to other forms of diabetic vascular disease; in particular, is CBMT observed in tissues amenable to needle biopsy, i.e., skeletal muscle, useful in identifying individuals at high risk for developing clinically significant retinopathy, nephropathy, or atherosclerotic vascular disease? In this survey, we will consider the nature of capillary basement membrane changes in diabetes and subsequently address the above questions.