Background & aims: Diabetes mellitus is an important risk factor for the development and progression of atrial fibrillation. There are several possible pathways to the mechanisms of atrial fibrillation associated with diabetes mellitus. Long-term inflammation is one of the mechanisms. To our knowledge, there is not a study that consistent evidence about the link between atrial fibrillation and neutrophil to lymphocyte ratio in diabetes mellitus. In the study, we aimed to investigate the possible association of chronic atrial fibrillation with neutrophil to lymphocyte ratio in patients who have type 2 diabetes mellitus.