For the purpose of detecting fetal pulmonary maturity with real-time convenience but reliable accuracy, infrared spectrophotometry of amniotic fluid lipids was introduced using our newly developed optical device. The peak areas at the wave number of 1740 cm-1 on the infrared spectra (called "PA1740") were measured in 29 specimens sampled from 29 pregnant women, and compared with the Lecithin/Sphingomyelin (L/S) ratio of the same samples. Statistical analysis revealed that the PA1740 was almost as reliable as the L/S ratio, and these indices illustrated pulmonary immaturity in cases with a PA1740 less than 0.9 and an L/S ratio less than 2.0. PA1740 is absolutely independent of the effect on the concentration of lecithin in the amniotic fluid because of a combination of the vacuum-drying process and the absorbance correction, and can be completed in less than 20 minutes from sampling of the amniotic fluid by a very simple manipulation. In conclusion, the new index, PA1740, will be a more practical method for prediction of the fetal pulmonary maturity than methods such as the well known L/S ratio.