The value of four amniotic fluid tests – the lecithin/sphingomyelin ratio, the presence of phosphatidylglycerol, amniotic fluid cytology and the creatinine concentration – in the prediction of fetal lung maturity was investigated. The sensitivity was high for all tests except for amniotic fluid cytology, which had a higher specificity. The overall outcome, particularly the value for predicting immaturity, was improved by various combinations of tests. The combination of amniotic fluid cytology and creatinine proved useful in predicting the gestational age. A diagram for estimation of gestational age is presented.