The surface areas of the juxtaglomerular cell complexes and the index of arteriolar sclerosis were evaluated in renal biopsies from 50 patients with diabetic glomerulosclerosis. The mean surface areas of the JGC's were significantly enlarged compared to that of normal renal tissue, whereas no significant difference could be found between kidneys from hypertensive and the normotensive diabetics. A positive linear correlation was found between mean JGC-size and the index of sclerosis of the afferent arterioles. This was interpreted as the result of faulty coordination between the activity of the juxtaglomerular apparatus and the systemic arterial blood pressure. This is caused by stenoses of the preglomerular afferent arterioles in diabetic arteriosclerosis.