Background: P-wave dispersion (PD), a measure of heterogeneity of atrial refractoriness, is defined as the difference between the minimum (P min) and maximum P-wave (P max) durations on standard 12-lead electrocardiography (ECG). Increase in PD shows the intra-atrial and inter-atrial non-uniform conduction. In the present study the evaluation of the effect of diabetes mellitus (DM) on PD in patients without coronary artery disease and hypertension was carried out. Methods and Results: Seventy-six diabetic patients who had no coronary artery disease or hypertension (group 1; mean age 48±9) and 40 healthy volunteer individuals (group 2; mean age 46±13) were enrolled in the study. After obtaining 12-lead surface ECG of all cases, P max and P min P-wave durations were measured and the differences between them were taken as PD (PD=P max-P min). Left atrium diameter, left ventricular end systolic and end diastolic diameters were measured and left ventricular ejection fraction was determined by echocardiography. Pulse wave mitral flow velocities were measured from the apical 4-chamber view. Mitral early diastolic velocity (E), late diastolic velocity (A), E/A, E deceleration time and isovolumetric relaxation time were determined. In comparison of the 2 groups there was no statistically significant difference among age, sex, systolic and diastolic blood pressure, resting heart rate and body mass index of the cases. Although PD and P max were significantly higher in diabetic patients, there was no difference between P min values (33±12 vs 28±10, p=0.02; 99±12 vs 93±10, p=0.011; 66±9 vs 65±10, p=NS; respectively). Conclusions: DM might increase PD even without ischemia, hypertension and left ventricular hypertrophy.