Abstract Background: Long term endurance training is known to increase peak oxygen uptake ( VO2peak ) and induce morphological changes of the heart such as increased left ventricular mass (LVM). However, the relationship between VO2peak and the total heart volume (THV), considering both the left and right ventricular dimensions in both males and females, is not completely described. Therefore, the aim of this study was to test the hypothesis that THV is an independent predictor of VO2peak and to determine if the left and right ventricles enlarge in the same order of magnitude in males and females with a presumed wide range of THV. Methods and Results: The study population consisted of 131 subjects of whom 71 were athletes (30 female) and 60 healthy controls (20 female). All subjects underwent cardiovascular MR and maximal incremental exercise test. Total heart volume, LVM and left- and right ventricular end-diastolic volumes (LVEDV, RVEDV) were calculated from short-axis images. VO2peak was significantly correlated to THV, LVM, LVEDV and RVEDV in both males and females. Multivariable analysis showed that THV was a strong, independent predictor of VO2peak (R2 = 0.74, p <0.001). As LVEDV increased, RVEDV increased in the same order of magnitude in both males and females (R2 = 0.87,p < 0.001). Conclusion: Total heart volume is a strong, independent predictor of maximal work capacity for both males and females. Long term endurance training is associated with a physiologically enlarged heart with a balance between the left and right ventricular dimensions in both genders.