The use of radiofrequency ablation for treatment of complex arrhythmia substrates has prompted interest in transcatheter endocardial activation mapping. Technical constraints on catheter fabrication and the intention to use such maps to guide ablation both demand innovative approaches to mapping. A fluoroscopically based endocardial mapping technique is proposed to improve the ability of electrophysiologists to interpret large amounts of data acquired using multielectrode catheter arrays, improving their ability to visualize the data and act on its content. This technique addresses previous limitations imposed by the number of electrodes that can be deployed and by the difficulty in determining their relative spatial locations. It is based on the composition of multiple activation sequence mappings made in a single rhythm, with the spatial locations of recording electrode pairs determined in orthogonal fluoroscopic views referenced to stable intrathoracic markers. Rather than imposing a geometry determined primarily by the measurement apparatus, the spatial locations of only those electrodes in proximity to the endocardial surface, as determined by their ability to record bipolar electrograms, are measured. In this manner, the geometry of the endocardium may be approximated by measurements made of electrode position. Using this approach, the number of endocardial sites that can be sampled in a stable rhythm is theoretically unlimited, resulting in the realization of high-resolution activation maps. Spatiotemporal data may be used to create three-dimensional activation sequence maps, displayed as animated sequences. This technique was used in anatomically normal and diseased human right atria to create activation maps of sinus and paced rhythms, classic atrial flutter, and postoperative intraatrial reentrant tachycardia, using a median of 108 electrode positions (range, 27-197) in 25 maps. The activation sequences represented by these maps were diverse, but qualitatively concordant with known mechanisms of atrial activation. High-density catheter-based activation mapping of the right atrium is feasible and may improve understanding of complex arrhythmias and assist in the development of ablative techniques. Further research is needed on the spatial correlation between cardiac anatomy and fluorography, suppression of spatial artifact, and optimal mapping densities.