BACKGROUND: Diabetes mellitus (DM) is a major risk factor for atherosclerosis. There is a controversy in literature about correlation between DM and atrial fibrillation. The goal of this study was to evaluate DM as a risk factor for atrial fibrillation or flutter using a very large database. METHOD: Patient treatment files (PTF) containing discharge diagnoses were utilized using ICD-9 codes of inpatient treatment from Veterans Health Administration Hospitals (VAH). Patients with type II DM (ICD-9 code 250.0) (293,124) discharged from the VAH between 1990 and 2000. Non-matched controls without DM but with hypertension (552,624) were selected from the same PTF. By using multi-variate logistic regressions, the occurrence of atrial fibrillation, atrial flutter, CHF, CAD and LVH was compared. RESULTS: Atrial fibrillations occurred in 43,674 (14.9%) DM patients vs. 57,077 (10.3%) in the control group (p<0.0001). Atrial flutter occurred in 11,852 (4%) of DM patients vs. 13,554 (2.5%) of the control group (p<0.0001). Using multi-variant analysis, DM remained independently associated with atrial fibrillation with an OR of 2.13, (95% CI: 2.10 to 2.16; p<0.0001) and flutter (OR 2.20, CI: 2.15 to 2.26; p<0.0001). Furthermore, CHF (OR 3.12, CI: 3.09 to 3.16; p<0.0001), LVH (OR 1.85, CI: 1.77 to 1.92; p<0.0001) and CAD (OR 2.39, CI: 2.34 to 2.44; p<0.0001) were also independently associated with DM. CONCLUSION: This is the first large-scale study finding DM as a strong, independent risk for the occurrence of atrial fibrillation and flutter and other cardiovascular disease.