To the Editor: Steffes et al. (May 16 issue) have recently documented that morphologic changes in renal tissue of patients with chronic insulin-dependent diabetes (IDD) do not occur in the nondiabetic identical twins of these patients.1 This finding provides further evidence that the microvascular disease seen in IDD is secondary to the metabolic derangements of the diabetic state, and not an isolated, foregone genetic conclusion. The authors wisely do not attempt to identify these metabolic derangements. Their failure to show a correlation between increased muscle capillary basement membrane width and fractional mesangial volume is disappointing. However, this apparent lack of.