The accurate estimation of glomerular filtration rate (GFR) is pivotal in sports medicine. However, there is controversial information on the acute influence of physical exercise on kidney function in healthy athletes. The estimated GFR (EGFR) was assessed by the recommended Modification of Diet in Renal Disease (MDRD) equation before a 21-km half-marathon, at the end, and 3, 6, 24 hrs thereafter on 17 trained, middle-aged males. Results were corrected for plasma volume changes. The mean EGFR at the baseline was 76 mL/min/1.73 m (2); it decreased at the end of the run (62 mL/min/1.73 m (2)) and for the following 3 hrs (68 mL/min/1.73 m (2)) and 6 hrs (70 mL/min/1.73 m (2)), though statistical significance was only achieved immediately after the run (mean decrease 16 %, p < 0.01). The frequency of athletes with EGFR below the normal threshold was higher than the baseline immediately after the race and for the following 6 hrs. Twenty-four hours after the run, the EGFR had returned to values similar and nonsignificantly different from those recorded at the baseline. These results attest that medium to high strains of running in healthy, middle-aged, trained individuals do not cause renal damage, but a limited and temporary decline in renal function.