Introduction: The maintenance of muscle mass is essential for both athletic and clinical populations. Creatine (methyl-d3) dilution (D3-creatine) is a novel technique for the estimation of muscle mass in humans and has recently been reported (Clark et al, 2014). The method uses a dose of deuterium-labelled creatine to determine total skeletal muscle mass via estimation of total body creatine pool size. This study is the first to apply this method in an athletic population. The aim of this study was to compare estimates of total body skeletal muscle mass from D3-creatine dilution method and whole body Magnetic Resonance Imaging (MRI) in an athletic population. Methods: Fifteen male and five female national level kayakers (stature: 182.0 ± 13.1 and 170.0 ± 9.0 cm; body mass: 80.6 ± 9.9 and 66.4 ± 6.0 kg; VO2max: 56.5 ± 7.0 and 49.6 ± 4.4 ml•kg-1•min-1 for males and females, respectively) underwent assessment at the start of pre-season training. Muscle mass was determined using two methods: MRI and D3-creatine. The D3-creatine technique required ingestion of two 30mg capsules and total urine was collected at 0-4 and 4-24 hour intervals over the next four days. Urine aliquots were analysed for creatine, creatinine, D3-creatine and D3-creatinine using liquid chromatography/mass spectroscopy (LC/MS/MS). Total creatine pool size was directly measured by enrichment of urinary D3-creatinine on day three, with a subsequent calculation for estimated total skeletal muscle mass. A Pearson’s correlation coefficient, using R statistical programme (Version 3.1.2, R), was used to analyse the association between creatine pool size and MRI methods. Results: The preliminary data demonstrates creatine pool sizes of 207.9 ± 36.0 g (mean ± S.D) and 144.0 ± 12.6 g for male and female athletes. Once the conversion factor for creatine pool size to muscle mass was implemented, an estimate of 43.6 ± 13.1 kg (57.41 ± 8.5 % of total body mass) was obtained. Muscle mass measured by MRI was on average 36.3 ± 6.2 kg (47.4 ± 4.3 % of total body mass). Estimated muscle mass was 7.3 kg (20%) higher on average using the D3-creatine technique than observed with MRI. Pearson’s correlation coefficient showed a strong correlation between D3 -creatine and MRI (r=0.953, p=<0.001). Discussion: This study is the first to apply the novel D3-creatine technique in an athletic population, for whom a convenient measure of muscle mass is required. The creatine pool sizes were larger than previously reported for young males by Clark and colleagues (187.6 ± 56.3 g vs 158.6 ± 26.9 g). D3-creatine over-estimated muscle mass in relation to MRI (8% – 28%). Possible explanations could include inexact MRI muscle mass estimates or inaccurate predicted concentration of creatine in whole wet muscle mass (4.3 g/kg; Kreisberg, 1970), required for muscle mass estimation from creatine pool size. Further work will be completed to improve the precision and accuracy of this tool within athletic cohorts.