Cardiac failure is a frequent feature in diabetic patients and it often causes their death. But how and when cardiac disease begins in this kind of patient is still debatable. For example, cardiac failure can be present even in the absence of atherosclerotic involvement of coronary arteries in young diabetics. The aims of our study were to evaluate the cardiac function and sympathetic tone of 16 young type 1 diabetic patients (8 M and 8 F, mean age: 27 years, SD +/- 5) in comparison with 10 normal subjects (4 M and 6 F, mean age: 30 years, SD +/- 7). Diabetic patients were choose from a large population because of the following features young age, absence of clinical and instrumental evidence of micro- or macroangiopathy, clinical evidence of diabetic autonomic neuropathy, proteinuria or arterial hypertension. They were in good metabolic control on daily insulin therapy of two or three administrations. Cardiac function was evaluated at rest and during submaximal exercise on a cycloergometer in supine position using radionuclide ventriculography with technetium 99m. Sympathetic tone was checked using the five clinical tests according to Ewing and the plasmatic level of catecholamines at rest was evaluated using high pressure chromatography. The ejection fraction, cardiac output, stroke volume of diabetics were comparable with those of normal subjects even in the presence of comparable systemic vascular resistance. The increase in ejection fraction during effort was normal. Only in one diabetic patient (incidentally the oldest one) did ejection fraction decrease (7%) during effort. The peak ejection and filling rates were significantly higher (p less than 0.001) in diabetic patients compared to those of normal subjects.(ABSTRACT TRUNCATED AT 250 WORDS)