The diagnostic value of roentgenometrically enlarged second arc of the anterior heart's shadow outline, a roentgenologic sign of right ventricular hypertrophy, is discussed. A study of 30 patients with various acquired and congenital valvular defects showed the enlargment of this arc, which is normally not more than 3.5 cm long and 0.5 cm wide, to be a reliable sign of right ventricular hypertrophy, as verified by ECG findings, and direct as well as the second oblique roentgenographic projections. Three degrees of the enlargment of the second arc in the heart's shadow anterior outline were distinguished in the first oblique projection. This arc is not evident in patients with electrocardiographic signs of right ventricular diastolic overloading.