In the early 1980s, studies in type 1 diabetes suggested that glomerular filtration rate (GFR) loss begins with the onset of macroalbuminuria. However, recent evidence indicates that up to one-quarter of subjects with diabetes reach a GFR of less than 60 ml/min/1.73 m2 (chronic kidney disease [CKD] stage 3) before developing micro- or macroalbuminuria. Furthermore, the prospective loss of GFR can be detected in early diabetic nephropathy (DN) well before CKD stage 3. Early GFR loss usually reflects DN in type 1 diabetes but, in older patients with type 2 diabetes, the assessment of early GFR loss needs to take into account the effects of aging. The assessment of GFR is now feasible at clinical level, using formulas based on serum creatinine, age, gender, and ethnicity. Overall, the estimation of early GFR loss is more accurate with the Chronic Kidney Disease Epidemiology (CKD–EPI) formula than with the Modification of Diet in Renal Disease (MDRD) study formula, but there is some evidence that the CKD-EPI formula does not exhibit better performance than the MDRD formula for estimating GFR in diabetes. Both formulas underestimate GFR in the hyperfiltration range. Formulas based on the reciprocal of cystatin C can also be used to estimate GFR, but their cost and lack of assay standardization have delayed their use at clinical level. In summary, early GFR loss is an important marker of DN as well as a potentially reversible target for interventions in DN.