Background: Atrial fibrillation is the most common type of heart rhythm disorder in clinical practice and its prevalence is on the rise worldwide. Obesity is one of the main risk factors for the appearance of atrial fibrillation. For these reasons, this paper aims to investigate the association of elevated body mass index, which is the most commonly used anthropometric measure for determining the degree of obesity, with the diagnosis of atrial fibrillation. Aim: This paper aims to investigate the association between atrial fibrillation and elevated body mass index, and other changes in health status associated with elevated body mass index, e.g. hypertension. Subjects and methods: A total of 268 patients of all ages participated in the study. The study included patients diagnosed with atrial fibrillation of both sexes, of which 138 were males and 130 females. Out of the total sample, 180 patients had a previous diagnosis of atrial fibrillation, while in 88 of them this was the first episode of atrial fibrillation. The medical database of the Polyclinic for the Prevention of Cardiovascular Diseases and Rehabilitation in Zagreb was used as an instrument of this research. Data were processed in the IBM SPSS Statistical Program Version 26.0. Differences in variables were tested by t-test and Benjamini -Hochberg correction of the obtained data. Results: Of the 268 subjects, 88 were in sinus rhythm at the first examination, and at the next examination in 2020, they were diagnosed with atrial fibrillation. The obtained results indicate that there was a significant increase in systolic pressure of patients with a previous diagnosis of sinus rhythm in the period from the first diagnosis to the examination in 2020. Histograms, box plots, and main parameters of distribution indicate a possible difference in systolic pressure at the first examination and the 2020 examination (i.e. an increase in systolic pressure in the period from the previous examination to the examination in 2020) for all patients, especially for patients diagnosed with sinus rhythm at the first examination and especially for male patients. Conclusion: The data do not indicate a statistically significant difference in systolic pressure between the first diagnosis and the 2020 review, or the statistically significant BMI difference between the first diagnosis and the 2020 review. There was a significant increase in systolic pressure from male patients between the first diagnosis and the examination in 2020 and there was a significant increase in systolic pressure of patients with a previous diagnosis of sinus rhythm in the period from the first diagnosis to the examination in 2020. There is no statistically significant difference in systolic pressure at the first examination between patients diagnosed with sinus rhythm and patients diagnosed with atrial fibrillation.