Glomerular hyperfiltration (>140 ml/min per 1.73 m2 body surface area) is found in early diabetes and is associated with a poor prognosis with respect to the development of diabetic kidney disease. This review addresses recent investigations and discussions of the following hypotheses behind diabetic hyperfiltration: Increased proximal tubular volume reabsorption results in a pressure drop in Bowman's capsule which increases glomerular filtration rate (GFR). Proximal tubular hyperreabsorption induces an increase in GFR mediated by tubuloglomerular feedback. Dietary NaCl restriction results in a paradoxically increased GFR and increased urine volume in diabetic animals.