To study the impact of well-controlled, uncomplicated maternal diabetes on fetal cardiac development and performance. The following variables were studied in 45 fetuses of type I diabetic women by means of mid- and late-trimester echocardiography: interventricular septal thickness; aortic and pulmonary valve diameters; peak and time-to-peak flow velocity of the great arteries; the ratio between peak velocities during early (E) and late (A) ventricular filling at the level of the atrioventricular values; ventricular fractional shortenings; and output. The findings were compared to age-matched control groups of normal fetuses. A significant augmentation of interventricular septal thickness was demonstrated for mid-trimester fetuses of diabetic women, which progressed further towards the end of pregnancy. However, the indices of diastolic and systolic function remained comparable between the gestational age-matched groups. Progressive myocardial thickening occurs commonly in mid- and late-trimester fetuses of uncomplicated and well-controlled diabetic pregnancies. The observed degree of hypertrophy is generally mild and does not affect age-related changes in fetal cardiac function.