The interpretation of biochemical testing in sportsmen requires caution. Although creatinine-based estimates of glomerular filtration rate (GFR) overcome some shortcomings of serum creatinine, there is scarce information on their use in endurance athletes.We evaluated GFR, estimated by the recommended Modification of Diet in Renal Disease (MDRD) equation in athletes.Seventy-six professional male cyclists, 71 amateur male cyclists, and 65 healthy sedentary matched controls were included in the study.The mean serum creatinine level was significantly higher in the sedentary subjects (81 microM) than in amateur (75 microM; P < 0.001) and professional cyclists (72 microM; P < 0.001), and it was also marginally higher in amateur than in professional cyclists (P = 0.049). The mean estimated GFR value increased throughout the three subgroups, being significantly lower in the sedentary population (98 mL.min.[1.73 m]) than in the subgroups of amateur (109 mL.min.[1.73 m]; P < 0.001) and professional cyclists (113 mL.min.[1.73 m]; P < 0.001), but it did not differ between amateur and professional cyclists (P = 0.116). The average intensity of daily physical exercise, but not the body mass index, was inversely associated with serum creatinine and positively associated with the estimated GFR.The MDRD equation should be used with caution in athletes, and it should consider intensity and type of physical exercise.