Fetal pulmonary maturity evaluation was performed on 237 amniotic fluid specimens from 202 gravidas using: 1) optical density (OD) 650, 2) shake or foam stabilization test, and 3) lecithin:sphingomyelin ratio. One hundred thirty-one gravidas delivered within 48 hours of specimen collection, including nine of 11 women delivering respiratory distress syndrome-affected infants and 40 delivering premature infants. The efficiency of each test in predicting fetal pulmonary maturity status was calculated and showed that statistically, OD 650 was significantly better at predicting fetal pulmonary maturity in preterm infants and all infants. Twenty-six of 27 infants of diabetic mothers (classes A through R) had mature OD 650 values; none developed respiratory distress syndrome. The authors confirm that the OD 650 is reliable in most clinical settings and superior to the shake test. The authors recommend its primary use when sophisticated phospholipid studies are not immediately available.