To the Editor: Seaquist et al. (May 4 issue)* observed that 83 percent of the diabetic siblings of diabetic probands with nephropathy also have nephropathy and that 83 percent of the diabetic siblings of diabetics without nephropathy are free of nephropathy. It is difficult to see how genetic factors can account for the clustering *Seaquist ER, Goetz FC, Rich S, Barbosa J. Familial clustering of diabetic kidney disease: evidence for genetic susceptibility to diabetic nephropathy. N Engl J Med 1989; 320:1161–5. of nephropathy observed in diabetic siblings by these authors unless nephropathy-risk alleles are genetically linked to mutations that confer.