Effective ablation of atrial fibrillation and other cardiac arrhythmias requires precise catheter navigation and controlled delivery of energy to cardiac tissue. In this study, we summarize our initial experience using a fiber optic direct visualization catheter to evaluate and guide placement of endocardial radiofrequency (RF) ablation lesions. RF lesions were created in cadaveric porcine hearts and examined in a blood-filled field using a direct visualization catheter. Direct visualization of RF lesions was repeated in vivo using an ovine model. Lesions and interlesion gaps were clearly identifiable using the direct visualization catheter. It was possible to place lesions in proximity to anatomical landmarks and in relation to one another. Catheter-generated images correlated well with lesion appearance on gross examination. Direct catheter-based visualization is a feasible technique for guiding RF lesion placement, estimating lesion size, and identifying interlesion gaps. Future work is needed to correlate surface appearance with transmurality and electrical isolation.