Heart size can be determined by a simple radiologic method here described. The method, widely used in Europe, depends on 3 measurements made on standard posteroanterior and lateral chest films at focal distances from 150 to 200 cm. The heart volume is calculated by slide rule from these data. Correlations of radiologically determined heart volume with other anatomic and physiological variants, such as calculated body surface area, total hemoglobin content, and physical working capacity, provide indices for recognizing cardiac enlargement and other cardiophysiologic abnormalities. This method has theoretical and practical advantages, and the author believes it to be the most uniform and precise plan for utilizing rediologic data for serial determinations of heart size.