Background: To assess left ventricular volumes and mass by cardiac magnetic resonance imaging in relation to conventional cardiovascular risk factors and coronary atherosclerotic plaque burden in master marathon runners aged ≥50 years. Methods: Cardiac MRI was performed in 105 clinically healthy male marathon runners (mean age 57.3 ± 5.7 years, range 50-71 years) on a 1.5 T MR system (Avanto, Siemens, Germany). Cine steady state free precession images in standard long and short axes views were acquired to assess left ventricular volumes and mass. Cardiovascular risk factors (blood pressure, HDL/LDL cholesterol, smoking, body mass index) were assessed and coronary artery calcification (CAC) was quantified by electron beam computed tomography. Results: Left ventricular muscle mass (mean LVMM = 140 ± 27 g; 73 ± 13 g/m2) increased with increasing left ventricular end-diastolic volume (mean LVEDV = 137 ± 32 ml; 72 ± 15 ml/m2) (r = 0.41, P < 0.0001) and with systolic (r = 0.33, P = 0.005) and diastolic (r = 0.28, P = 0.005) blood pressures. Left ventricular EDV increased up to the age of 55 years, but decreased thereafter. Runners with LVMM ≥150 g had significantly higher CAC scores than runners with LVMM <150 g (median CAC score 110 vs. 25, P = 0.04). Conclusions: Increases in LVMM and LVEDV may not only represent a response to exercise but are dependent on age and blood pressure, also. In addition, a left ventricular hypertrophy without an increase in volume may be an indicator for early subclinical cardiac alterations in response to risk factor exposure.