Although non-invasive studies in type I diabetic subjects indicate left ventricular (LV) diastolic dysfunction, the contribution of borderline or mild hypertension to such changes is obscure. Thus, digitized M-mode echocardiograms were obtained in 32 (18 men) young (less than 50 years) normotensive controls and 32 (21 men) long-term (greater than or equal to 12 years) type I diabetics with blood pressures ranging from normal to hypertensive. All diabetics were without clinical heart disease, none were previously treated for hypertension or using cardioactive drugs. Heart rate, systolic and diastolic blood pressures were higher in diabetic than control subjects. Their LV end-diastolic dimension was smaller, whereas wall thickness, LV mass index and fractional shortening were similar to controls. In diabetics, however, the normalized peak filling rate was decreased and the rapid filling period fraction of diastole increased. In multivariate analysis, diabetes and LV mass independently and inversely influenced the normalized peak filling rate, while fractional shortening did so positively. Furthermore, diabetes and systolic blood pressure independently influenced the rapid filling period fraction of diastole. This study is the first to demonstrate systolic blood pressure and LV mass as independent contributors to subclinical LV diastolic abnormalities in diabetics. These findings may therefore indicate the need to treat even mild hypertension in diabetics in an effort to delay the development of cardiopathy.