A formula to estimate heart weight was derived using a theory of quantification which employed the measurements of cardiac silhouette taken from routine postero-anterior projection chest X-ray film. The films obtained in 73 necropsied cases of essential hypertension, renal hypertension, mitral and aortic valvular diseases, and 62 necropsied cases with other diseases were used for derivation of the formula. The measurements of the horizontal distances from the midsternal line to the intersecting point of the lower margin of each rib along the left cardiac border, and of the greatest distance along the right cardiac border were employed; body height, age and sex were also included. The heart weight estimated by this formula coincided with the heart weight measured at necropsy with a high correlation coefficient of 0.81. The correlation rate in 34 external samples was also significantly high, being 0.72. These results indicate the practical usefulness of this formula.