We examined renal-transplant tissue from 12 diabetic and 28 nondiabetic patients who had had a renal graft for at least two years. In 10 diabetic patients arteriolar hyalinosis lesions developed in the graft. In six these lesions involved both afferent and efferent limbs of glomerular arterioles - a pathological finding virtually diagnostic of diabetes mellitus. In all cases these lesions were present within five years of transplantation. Only three of the 28 nondiabetic patients had hyaline vascular changes (P less than 0.001), which occurred only in rare vessels, did not appear within the first five years after transplantation and did not involve both afferent and efferent arterioles, One diabetic patient had nodular glomerulosclerosis. Thus, the first clearly distinguishable lesion of diabetes to occur with frequency in normal kidneys transplanted into diabetic patients is arteriolar hyalinosis.