Samples [246] of amniotic fluid were obtained from 245 women with normal or complicated pregnancy near or at parturition. The lecithin/sphingomyelin (L/S) ratio was determined and retrospectively correlated with the clinical course of the neonate. Ten neonates developed hyaline membrane disease (HMD). The highest L/S-ratio in this group was 1.82. Cesarean section was not associated with HMD if L/S-ratio indicated pulmonary maturity (L/S > 2). With the exception of children born by women with toxemia or chronic hypertensive disease, where no case of HMD appeared among 9 infants, children with birth weights < 2000 g developed HMD in about 50% (6/11 cases). The results show that the L/S-ratio correlates well to the pulmonary maturity of the neonate. Determination of L/S-ratio is of great importance in cases of high risk pregnancy or doubtful gestational age prior to induction or Cesarean section.