A correlation between high insulin levels in amniotic fluid and the appearance of diabetogenic fetal morbidity was found in radioimmunoassays of 487 samples of the fluid. The mean insulin level in metabolically normal pregnancies rose from 9 muU/ml (Week 27) to 15 muU/ml (Week 40). The insulin level in amniotic fluid of diabetic patients was elevated up to 27 times the mean. Insulin levels in the umbilical cord and urine of newborns of diabetic mothers were also elevated, to 29 and 21 times the mean, respectively. Elevation of insulin levels in amniotic fluid portends diabetogenic fetal morbidity. High and rising insulin levels at an early stage (26-28 weeks) may indicate a high risk of preterm onset of labor. Regular adjustment of metabolic compensation on the basis of amniotic fluid insulin made it possible to reduce the level in 12 of 17 pregnant diabetic women by increasing insulin dosage. The 12 women were thereby enabled to carry their pregnancies to term and to await the onset of spontaneous labor without diabetogenic fetal morbidity. Beta-stimulating agents affect glucose management and may cause elevated insulin levels in amniotic fluid.