Pulmonary vein isolation (PVI) is considered a standard treatment of paroxysmal atrial fibrillation aiming to restore and maintain sinus rhythm. The purpose of this study is to analyse how the PVI treatment affects the electrical conduction pattern in the atria. We have compared the morphology of P wave loops extracted from the vectorcardiogram (VCG) before and after PVI. Ten patients suffering from paroxysmal atrial fibrillation who underwent PVI were included in the study. All patients were in sinus rhythm before as well as after the procedure. Vectorcardiogram was obtained using the Kors matrix and the resulting P-waves were automatically delineated. Using these marks, P-wave loops were constructed from the three orthogonal leads of the VCG. The first and second eigenvalues of the P-wave loops increased after the PVI procedure was completed, whereas the ratio of the second to the first eigenvalue, quantifying the roundness of the P-wave loop, decreased significantly after PVI. These results show that PVI induces morphological changes of the P-wave loops.