In pregnancies complicated by diabetes, foetal lung maturation depends on a good control of maternal blood glucose values. In poorly controlled maternal diabetes, foetal hyperinsulinaemia may cause a delay in pulmonary maturation. There was no single case of respiratory distress syndrome (RDS) in 112 pregnant class B-F diabetic patients that we treated with high doses of insulin. Furthermore, in a controlled randomized trial of diet versus insulin treatment in class A diabetes we found no differences in perinatal mortality in patients with adequate metabolic control. In pregnancies complicated by Rh isoimmunization, determination of phosphatidylglycerol in amniotic fluid is a more reliable marker of foetal lung maturity than is the lecithin/sphingomyelin ratio. The marked decrease in perinatal mortality due to Rh incompatibility observed in recent years depends on several factors including administration of corticosteroids to the mother to prevent RDS, irrespective of whether amniotic fluid parameters indicate foetal lung maturity.