Aims/hypothesis GFR is commonly estimated using the four-variable Modification of Diet in Renal Disease (MDRD) formula and this forms the basis for classification of chronic kidney disease (CKD). We investigated the effect of obesity on the estimation of glomerular filtration rate in type 2 diabetic participants with CKD. Methods We enrolled 111 patients with type 2 diabetes mellitus in different stages of CKD. GFR was measured using 51 Cr-labelled EDTA plasma clearance and was estimated using the four-variable MDRD formula. Results The bias between estimated and measured GFR was −22.4 (−33.8 to −11.0) p  < 0.001 in the obese group compared with −6.04 (−17.6 to −5.5) p  = 0.299 in the non-obese group. When GFR was indexed to body surface area of 1.73 m 2 , the bias remained significant at −9.4 (−13.4 to −5.4) p  < 0.001 in the obese participants. Conclusions/interpretation This study suggests that the four-variable MDRD formula significantly underestimates GFR in obese type 2 diabetic participants with CKD.