In 142 diabetic pregnancies fetal biparietal diameter (BPD), transverse abdominal diameter (TAD) and abdominal circumference (AC) were estimated by means of serial ultrasound examinations. An accurate evaluation of fetal growth was obtained in 85% of the cases. AC was found to have the highest degree of reliability in predicting macrosomia (71%) followed by TAD (60%) and BPD (17%). Placental morphology was estimated at the last scanning before delivery in 85 of the patients. In pregnancies resulting in macrosomic infants the placental maturation seemed to be retarded. In 26 patients a third trimester amniocentesis was performed. A mature (grade III) placenta was always associated with a lecithin-sphingomyelin (L/S) ratio greater than 2, indicating fetal lung maturity but there was no constant relation between placental maturity and the L/S ratio. In conclusion, ultrasound examination during diabetic pregnancy offers valuable information concerning the prenatal diagnosis of macrosomia and could serve as a guidance in the determination of the optimal time and route of delivery. The grading of placental maturity might help to identify fetuses at risk, but further studies in this area are needed before final conclusions can be drawn.