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Background: Creatinine concentrations in athletes could be higher than in sedentary people due to higher muscle mass; a correlation between body mass index (BMI) and creatinine concentrations has been described only in male athletes participating in different sports. Methods: Elite swimmers (28 males and 26 females) belonging to the National Teams of Slovenia were recruited during the period 1999-2009. The evaluation of possible modifications to serum creatinine concentrations during a single season were performed in 10 males and seven females who were evaluated four times during a season. Serum creatinine was analyzed using the Jaffe method. Pearson's simple correlation analysis was used to test the association between serum creatinine and BMI. Creatinine values and BMI were compared using the unpaired Student t-test. Creatinine values during the different phases of the season were compared using a one-way analysis of variance test. Results: The values for creatinine are related to BMI in male and female swimmers. It should be noted that there is a significant difference (p<0.001) for estimated glomerular filtration rate (eGFR) values obtained by Cockcroft and Gault equation, but not for the Modification of Diet in Renal Disease (MDRD) equation. The values of creatinine were stable in each athlete during various training cycle periods in the observed competition year. Conclusions: The study confirms that in a large and representative group of elite swimmers, creatinine concentration is strictly correlated to BMI. The use of creatinine-based eGFR formulas should be used with caution in athletes. [ABSTRACT FROM AUTHOR]