Maintenance of adequate renal function after living kidney donation is important for donor outcome. Overweight donors in particular may have an increased risk for end stage kidney disease (ESKD), and young female donors have an increased preeclampsia risk. Both of these risks may associate with low post-donation renal functional reserve (RFR). Because we previously found that higher BMI and lower post-donation RFR were associated, we now studied the relationship between BMI and RFR in young female donors. RFR, the rise in GFR (125I-Iothalamate clearance) during dopamine, was measured in female donors (<45 years) before and after kidney donation. Donors who are overweight (BMI>25) and non-overweight donors were compared by t-test; the association was subsequently explored with regression analysis. We included 105 female donors (age 41 [36-44] (median[IQR])) with a BMI of 25 [22-27] kg/m2. Pre-donation GFR was 118 (17) ml/min (mean(SD)) rising to 128 (19) ml/min during dopamine; mean RFR was 10 (10) ml/min. Post-donation GFR was 76 (13) ml/min, rising to 80 (12); RFR was 4 (6) ml/min (p<0.001 vs. pre-donation). In overweight donors, RFR was fully lost after donation (1 ml/min vs. 10 ml/min pre-donation, p<0.001), and BMI was inversely associated with RFR after donation, independent of confounders (St. β 0.37, p=0.02). Reduced RFR might associate with the risk of preeclampsia and ESKD in kidney donors. Prospective studies should explore whether RFR is related to preeclampsia and whether BMI reduction prior to conception is of benefit to overweight female kidney donors during and after pregnancy.