OBJECTIVE: To identify the presence of ventricular diastolic dysfunction by tissue Doppler in fetuses of diabetic mothers, with or without septal hypertrophy, in comparison to fetuses of nondiabetic mothers. METHODS: A contemporary transverse study in fetuses with a gestational age between 25 weeks to term, studying diastolic function by assessment using tissue Doppler and pulsed wave Doppler of the atrioventricular diastolic flow. The mothers of the fetuses all had previous or gestational diabetes, and were referred to the Fetal Cardiology Unit of the Institute of Cardiology in Porto Alegre, Brazil. We analysed variance with the Student-Neumann-Keuls post hoc test. An alfa of 0.05 was considered significant for statistical analysis. RESULTS: The mean myocardial velocities of the E'and A' waves at the mural mitral annulus, in fetuses of diabetic mothers with myocardial hypertrophy, were, respectively, 7.00 plus or minus 1.6 centimetres per second, and 10.24 plus or minus 3.3 centimetres per second. In the fetuses of diabetic mothers group without myocardial hypertrophy, the comparable values were 7.19 plus or minus 2.4 centimetres per second and 10.77 plus or minus 3.77 centimetres per second, respectively. In the control group, they were 4.81 plus or minus 0.85 centimetres per second and 8.01 plus or minus 2.2 centimetres per second. The difference between the velocities in fetuses of diabetic mothers and in fetal normal mothers was statistically significant (p less than 0.05). Statistically significant differences were also observed in E' and A' diastolic waves at the aortic mitral annulus, as well as for the tricuspid annulus when tissue Doppler assessment was carried out in the same sample. The mean ratio between the E and E' of mitral and tricuspid waves in the control fetuses of normal mothers was significantly higher than in fetuses of diabetic mothers. CONCLUSION: Pulsed tissue Doppler, when used in fetuses of diabetic mothers and compared with fetuses of nondiabetic mothers, shows evidence of impaired diastolic function, independently of the presence of myocardial hypertrophy.