The aim of the study was to analyze right ventricular (RV) myocardial function in patients with left ventricular (LV) hypertrophy secondary to either hypertrophic cardiomyopathy (HC) or athletic endurance training. Doppler echocardiography and pulsed Doppler tissue imaging of the posterior septal wall, and mitral and tricuspid annulus were performed in 32 top-level endurance athletes (AT) and in 27 patients with HC, all men. LV mass index was comparable between the 2 groups. All transmitral Doppler indexes were higher in AT, whereas only tricuspid inflow peak E and E/A ratio were slightly decreased in the HC group. In the HC group, Doppler tissue analysis showed lower myocardial systolic and early-diastolic (Em) peak velocities, and longer time intervals at the level of all the analyzed segments, even after correction for age, heart rate, and LV mass index. Distinct multiple linear regression models revealed an independent positive association between RV peak Em velocity and LV end-diastolic diameter (beta coefficient = 0.72, P <.0001) in AT, and an independent inverse correlation of the same peak Em velocity of tricuspid annulus with septal thickness (beta = - 0.65, P <.001) in the HC group. Of interest, a RV Em peak velocity < 0.16 m/s differentiated AT and HC groups better than tricuspid Doppler (89% sensitivity and 93% specificity). In conclusion, Em RV myocardial function is positively influenced by preload increase in AT and negatively associated to increased septal thickness in patients with HC. Therefore, Doppler tissue imaging may represent a useful tool in the differential diagnosis between athlete's heart and HC, underlining the different involvement of RV myocardial function in either physiologic or pathologic LV hypertrophy.