Background: There is increasing interest in performing left atrial appendage (LAA) occlusion at the time of atrial fibrillation (AF) ablation procedures. However, to date there has been no description of the acute changes to the LAA immediately following pulmonary vein (PV) isolation and additional left atrium (LA) substrate modification. This study assessed changes in the size and tissue characteristics of the LAA ostium in patients undergoing PV isolation. Methods: This series included 8 patients who underwent cardiovascular magnetic resonance evaluation of the LA with delayed enhancement magnetic resonance imaging and contrast enhanced 3-D magnetic resonance angiography pre-, within 48 h of, and 3 months post ablation. Two independent cardiac radiologists evaluated the ostial LAA diameters and area at each time point in addition to the presence of gadolinium enhancement. Results: Compared to pre-ablation values, the respective median differences in oblique diameters and LAA area were +1.8 mm, +1.7 mm, and +0.6 cm2 immediately post ablation (all NS) and −2.7 mm, −2.3 mm, and −0.5 cm2 at 3 months (all NS). No delayed enhancement was detected in the LAA post ablation. Conclusion: No significant change to LAA diameter, area, or tissue characteristics was noted after PV isolation. While these findings suggest the safety and feasibility of concomitant PV isolation and LAA device occlusion, the variability in the degree and direction of change of the LAA measurements highlights the need for further study.