Patients with Type 1 (insulin-dependent) diabetes show reduced skeletal muscle blood flow and coronary vasodilatory function despite intensive insulin therapy and good metabolic control. Administration of proinsulin C-peptide increases skeletal muscle blood flow in these patients, but a possible influence of C-peptide on myocardial vasodilatory function in Type 1 diabetes has not been investigated. Ten, otherwise healthy, young male Type 1 diabetic patients (HbA_1c 6.6 %, range 5.7- 7.9) were studied on two consecutive days during normoinsulinemia and euglycemia in a doubleblind, randomized, cross-over design, receiving intravenous infusion of C-peptide (5 pmol^.kg^{-1 .}min^-1) for 120 min on one day and saline infusion on the other day. Myocardial blood flow (MBF) was measured at rest and during adenosine administration (140 µg^.kg^{-1 .}min^-1) both before and during the C-peptide or saline infusions using positron emission tomography and [¹⁵O]H₂O administration. Basal MBF was not significantly different in the patients compared to an age matched control group, but adenosine-induced myocardial vasodilation was 30% lower (p<0.05) in the patients. During C-peptide administration adenosine-stimulated MBF increased on average 35% more than during saline infusion (P<0.02) and reached values similar to those for the healthy controls. Moreover, as evaluated from transthoracal echocardiographic measurements, C-peptide infusion resulted in significant increases in both left ventricular ejection fraction (+5%, p<0.05) and stroke volume (+7%, p<0.05). It is concluded that short-term C-peptide infusion in physiological amounts increases the hyperemic myocardial blood flow and left ventricular function in Type 1 diabetic patients.