Fifty‐one strictly controlled pregnancies in 49 insulinrequiring diabetic women were studied. During the last trimester serial analyses of maternal urinary oestriol excretion were performed. A non‐intervention programme was adopted, so that the pregnancies were not interrupted before term unless necessary. Serial analyses of maternal oestriol excretion were of great value for the supervision of the pregnancies and for the timing of the delivery provided that the oestriol values were obtained within 6–8 hours after sampling. No intrauterine death occurred and the perinatal mortality was 4%. The urinary oestriol excretion in these diabetic pregnancies corresponded to that in normal pregnancies. No significant differences in excretion were found when pregnancies in women with diabetes belonging to White classes B and C were compared to those with diabetes belonging to class D. Nor was any difference in urinary oestriol excretion found when comparing preeclamptic with non‐preeclamptic pregnancies. There was a significant correlation between infant birth weight at a given gestational age and the maximum maternal urinary oestriol excretion.