Beginning in 1963 we have administered fast-acting insulin in three daily injections up to the maximal tolerated dose (M.T.D.), which can be defined as the highest quantity that can be given without bringing about hypoglycemic disturbances. This therapeutic criteria was applied both to gestational (280 pregnant women) and clinical (199 pregnant women) diabetes. M.T.D. was established on first admission to hospital and afterward controlled weekly in the outpatient clinic and during short periods of hospitalization (average of 53 days per patient). The average increase of the M.T.D. was 38 IU from 15 wk until delivery. As regards maternal blood sugar, at the M.T.D. average values at fasting and over the 24-h period were very close to the corresponding values of the control group (normal pregnancy). Total perinatal mortality (P.M.) was 2.9%; in the gestational diabetes group it was 2%. The incidence of congenital malformations (C.M.) (2.4%) increased with the severity of diabetes. No congenital defects were observed in 96 infants of patients treated before the 15th wk. Labor started spontaneously in 90.4% of the cases. The incidence of cesarean section was 21% in patients in White's Classes Al and A2 (as recently redefined by Freinkel and Metzger8) and 28% in other classes and that of forceps and vacuum extraction was 1% and 6%, respectively. The incidence of small-for-dates and overweight newborns from mothers with gestational and clinical diabetes without vascular complications is consistent with normal figures (macrosomia 3.4%). Small-for-dates newborns from pregnant diabetic women with vascular complications had a significant incidence. Respiratory distress syndrome (RDS) was 0.6% and hypoglycemia was 13.4%. No case of death was due to these two causes. Clinical results are discussed.