The amylase concentration in the amniotic fluid (ACAF) was studied in 138 subjects in relation to duration of pregnancy, fetal weight, L/S ratios and neonatal occurrence of respiratory distress syndrome (RDS). ACAF was found to be significantly correlated with duration of pregnancy (p < 0.001) and fetal weight (p < 0.005). The number of false immature predictions, based on a critical value of ACAF of 200 U/l, was relatively high: 36% for the gestational age ≥ 36 weeks, and 38% for fetal weight > 2,500 g. ACAF also showed a significant relationship with the L/S ratio (p < 0.0001). ACAF < 300 U/l correctly predicted all instances of L/S ratio < 2.0; however, the rate of false low predictions was 60%. In only 22 out of 138 AF samples (16%) could the performance of an L/S ratio have been avoided on the basis of the ACAF. ACAF < 300 U/l correctly predicted both cases of neonatal RDS in the study population; however, 1 of these fetuses had an ACAF of 260 U/l. There were four times as many false predictions of lung immaturity on the basis of ACAF < 300 U/l as from the L/S < 2.0.