To analyse the anatomy and systolic and diastolic cardiac function in a group of type I diabetics with no other abnormality and to correlate it with the duration of the disease, the presence of complications, the control of the diabetes and the abnormalities in the autonomous nervous system, 125 type I diabetics and 50 age- and sex-matched healthy controls were studied. In 112 diabetics, an echocardiographic image which enabled us to calculate the thickness, cavity dimensions and systolic function rates was obtained. A Doppler echocardiograph was done in all patients to measure 9 parameters of diastolic function. The autonomic nervous system was evaluated by the response to 4 cardiovascular reflexes. Two control groups and 4 study groups were established, based on duration and on the presence and number of microangiopathic complications. The results showed a significant increase in the septal and posterior wall thickness, although without differences between the study groups. There were no differences in the analysis of systolic function. The abnormalities in diastolic function were significant in all the groups, but greater in the groups with microangiopathy. Overall, for groups 1-4, respectively, the incidence of anatomical abnormalities was 9.6%, 17%, 28% and 57% (average 22%); systolic 0%, 0%, 4% and 4.7% (average 2.2%); and diastolic 15%, 21%, 60% and 80% (average 44%). Only 13 diabetics from group 4 presented with cardiac autonomic neuropathy. No correlation between these alterations and the glycaemic control or the duration of the disease was found, although there was a correlation between the presence or absence of complications and the anatomic and diastolic abnormalities.(ABSTRACT TRUNCATED AT 250 WORDS)