Objectives: Extensive endurance training and arterial hypertension are established risk factors for atrial fibrillation. We aimed to assess the proportion of masked hypertension in endurance athletes and the impact on cardiac remodeling, mechanics, and supraventricular tachycardias (SVT).Methods: Male participants of a 10-mile race were recruited and included if office blood pressure was normal (<140/90 mmHg). Athletes were stratified into a masked hypertension and normotension group by ambulatory blood pressure. Primary endpoint was diastolic function, expressed as peak early diastolic mitral annulus velocity (E'). Left ventricular global strain, left ventricular mass/ volume ratio, left atrial volume index, signal-averaged Pwave duration (SAPWD), and SVT during 24-h Holter monitoring were recorded.Results: From 108 runners recruited, 87 were included in the final analysis. Thirty-three (38%) had masked hypertension. The mean age was 42 AE 8 years. Groups did not differ with respect to age, body composition, cumulative training hours, and 10-mile race time. Athletes with masked hypertension had a lower E' and a higher left ventricular mass/volume ratio. Left ventricular global strain, left atrial volume index, SAPWD, and SVT showed no significant differences between the groups. In multiple linear regression analysis, masked hypertension was independently associated with E' (beta ¼ -0.270, P ¼ 0.004) and left ventricular mass/volume ratio (beta ¼ 0.206, P ¼ 0.049). Cumulative training hours was the only independent predictor for left atrial volume index (beta ¼ 0.474, P < 0.001) and SAPWD (beta ¼ 0.481, P < 0.001). Conclusion:In our study, a relevant proportion of middleaged athletes had masked hypertension, associated with a lower diastolic function and a higher left ventricular mass/ volume ratio, but unrelated to left ventricular systolic function, atrial remodeling, or SVT.