With the increasing popularity of cardiac ablation therapy, studies of the procedural effects on left atrial and pulmonary vein morphology are becoming more important. Of particular interest is evaluation of atrial and pulmonary vein remodeling following ablation therapy using structural imaging. One challenge that arises when comparing pulmonary vein morphology across subjects is defining the ostial location. Strategies for defining this important anatomical location include volume renderings from multiple angles, or drawing lines in cross-sectional images. Drawbacks of these techniques include subjectivity between raters as well as limited use of three dimensional volumetric information. In this work, we describe a method for automatically identifying the pulmonary vein ostia from CT images using a single user-selected seedpoint. The technique makes use of the full three-dimensional volumetric information, by computing a centerline along each pulmonary vein and defining the ostium using oblique cross-sectional image planes along the curve axis. The ostium is defined as the point at which there is a spike in the oblique cross-sectional area. The method is demonstrated on each of the four pulmonary veins in four patient datasets, for a total of sixteen applications of the algorithm. The results are compared against manual delineations of the pulmonary vein ostia, with overall mean distances ranging from approximately 1.5 to 5.0 mm. In conclusion, although the pulmonary veins exhibit variable anatomic shapes and orientations across different patient datasets, our proposed automated method produces results comparable to manual delineation of the ostia.