The precise delineation of the left ventricular projection area is an essential part in the quantitative analysis of nuclear angiocardiograms. We have devised an algorithm that permits automation of this step, based on a one-dimensional Laplace operator whose kernel is 2, 2, -2, -4, -2, 2, 2. The operator characteristically enhances "valleys" more than edges and, therefore, favors septal and the valve plane detection. The operator is applied vertically, horizontally, and along both diagonals. Each pass is immediately followed by a local maximum search during which the image resulting from the Laplacian operator is reduced to a binary one, with zeros everywhere except where a local maximum was found along the path of the operator. This resultant image yields a closed "edge" around the left ventricle, even though many structures outside the left ventricle are also delineated. However, the centroid of the ventricle is defined from functional criteria and the region of interest is defined from centroid to first edge. The method has been applied to first-pass and gated studies in anterior and 45 degree left anterior oblique views. In 100 successive cases the ejection fraction obtained automatically was compared to the manual result. The regression equation yielded the relation: automatic method (%) = 1.7 + 1.0 manual method (%) +/-2% (r = 0.995), which is not significantly different from the identity relation. The failure rate was low (13%) but varied from 28% in the first-pass studies in the anterior view, to less than 8% in gated studies in the left ventricular oblique projection.