Several studies have demonstrated evidence for preclinical left ventricular (LV) diastolic dysfunction in patients with diabetes mellitus (DM) independent of coronary disease or hypertension. The objectives of our study were to determine if LV diastolic dysfunction determined by tissue Doppler indexes worsens with duration of DM and to quantify severity of dysfunction as a function of DM duration. From 1996 to 2007, all Olmsted County, Minnesota, residents with DM free of heart failure who had a subsequent measurement of diastolic function using tissue Doppler echocardiography were identified. We identified a validated group of 486 patients with incident DM with a subsequent tissue Doppler echocardiographic assessment of diastolic function. There was a significant association between the ratio of early mitral velocity (E) to medial mitral annulus velocity (e′) and time from diabetes diagnosis to echocardiogram using simple linear regression; for every 1 year after the onset of diabetes, E/e′ increased by 0.23 (95% confidence interval [CI] 0.16 to 0.30, p = 0.007) after adjustment for age, gender, body mass index, previous coronary disease, previous hypertension, and ejection fraction. A duration of diabetes ≥4 years was independently associated with LV diastolic dysfunction (E/e′ >15) in multivariable logistic regression modeling after adjustment for age, gender, body mass index, previous coronary disease, previous hypertension, and ejection fraction (odds ratio 1.91, 95% CI 1.19 to 3.07, p = 0.007). There were 48 deaths in the validated cohort (6 cardiac deaths). In multivariable proportional hazard modeling, E/e′ ratio was predictive of all-cause mortality after adjustment for age, gender, coronary disease, hypertension, ejection fraction, left atrial volume, and time from DM to echocardiogram (risk ratio 1.11, 95% CI 1.03 to 1.20, p = 0.005). In conclusion, duration of DM of ≥4 years is correlated with significant LV diastolic dysfunction. LV diastolic dysfunction is predictive of all-cause mortality in patients with DM independent of hypertension and coronary disease.