An abnormal left ventricular (LV) diastolic function is an early sign of diabetic cardiomyopathy, which is characterized by an impaired diastolic and/or systolic function of the left ventricle in the absence of ischemic, valvular, or hypertensive heart disease, and serves as a marker of cardiovascular risk. However, it is unclear whether LV diastolic abnormalities can be detected in patients with impaired glucose tolerance (IGT) or mild diabetes without LV hypertrophy (LVH). We examined echocardiographic data from 92 consecutive Japanese patients aged 45-79 years with or without IGT or mild diabetes in the absence of LVH. Impaired glucose tolerance or mild diabetes was defined as the presence of one or more of the following criteria: fasting plasma glucose >110 mg/dl, hemoglobin A1c >5.6%, homeostasis model assessment ratio >1.73, or the taking of oral antihyperglycemic drugs. Left ventricular hypertrophy was defined as an LV mass index (LVMI) >116 g/m(2) in men and >104 g/m(2) in women. Patients with ischemic, valvular, or hypertensive heart disease were excluded. The age, blood pressure, heart rate, and LVMI were similar between patients with (IGT/DM group, n = 43) and without IGT or mild diabetes (non-IGT/DM group, n = 49), whereas the body mass index and waist circumference (WC) were greater in the IGT/DM compared to the non-IGT/DM group (P < 0.05 and P < 0.001, respectively). The transmitral E/A ratio was lower and the deceleration time longer in the IGT/DM than in the non-IGT/DM group (both P < 0.05). Stepwise regression analysis revealed that age and WC were independent determinants of the E/A ratio. In conclusion, diastolic abnormalities without LVH can be detected in Japanese patients with IGT or mild diabetes. The E/A ratio decreases in association with abdominal fat accumulation.