Atrial fibrillation (AF) is a common disease with a poorly understood pathophysiological mechanism. Increasing evidence indicates that AF may be associated with immunologic inflammation responses, but it remains unclear whether activation of peripheral blood CD3(+) T-lymphocytes plays a role in the pathogenesis of AF. The aim of this study was to evaluate this phenomenon. Fifty paroxysmal AF patients and 56 persistent AF patients who underwent successful electrical cardioversion were enrolled. The percentages of CD69 and human leukocyte antigen DR (HLA-DR) positive peripheral blood CD3(+) T-lymphocytes, which indicate T-lymphocyte activation, were examined by flow cytometric analysis in the patients and 51 healthy controls. The patient groups had higher levels of CD69 and HLA-DR than the healthy controls. During the 3-month follow-up, 37 patients had recurrence of AF (recurrence group) and 50 patients remained in sinus (sinus group). The CD69 and HLA-DR levels in the sinus group were all significantly down-regulated at follow-up compared with before cardioversion. However, there were no statistically significant differences between the CD69 and HLA-DR levels in the recurrence group at follow-up and before cardioversion. Our findings suggest that activation of peripheral blood CD3(+) T-lymphocytes was associated with AF, and might be a diagnostic or therapeutic marker.