A non-invasive Doppler ultrasound technique, based on the measurement of pulse wave velocity along the aorta, has been used to deduce aortic compliance in 25 Type 1 and 25 Type 2 diabetic patients. Thirteen of the Type 1 diabetic group had their compliance measured within 1 year of diabetes first being clinically diagnosed. All compliance values were normalized for age and sex variations using data previously obtained from over 600 normal, non-diabetic subjects (mean normalized compliance +/- SD; 100 +/- 15%). The results show that Type 1 diabetic patients have significantly more distensible aortas (132 +/- 26%) than their age- and sex-matched non-diabetic counterparts (100 +/- 12%) (p less than 0.01), while Type 2 diabetic patients have significantly stiffer aortas (74 +/- 21%) than their age- and sex-matched non-diabetic counterparts (100 +/- 18%) (p less than 0.01). The young Type 1 diabetic patients measured within 1 year of diagnosis have aortas ranging up to 78% more distensible (151 +/- 15%) than their age- and sex-matched non-diabetic controls (100 +/- 11%) (p less than 0.001). These results support findings by other groups that adult diabetic patients have less distensible arteries than normal, but contradict reports in the literature dating back over 20 years that diabetic children have stiffer arteries than normal children.