OBJECTIVE We evaluated the usefulness of the recently introduced TDx FLM assay in determining fetal lung maturity. STUDY DESIGN The TDx FLM assay was compared with the lecithin/sphingomyelin ratio, the foam stability index, and the presence of phosphatidylglycerol in amniotic fluid samples from 102 pregnancies, among which respiratory distress syndrome developed in 22 infants. RESULTS The TDx FLM assay met or exceeded results of other tests with respect to sensitivity (100% vs 86% to 100%), specificity (82% vs 38% to 78%), predictive value of a mature test result (100% vs 95% to 100%), predictive value of an immature test result (61% vs 31% to 51%), and efficiency (86% vs 51% to 79%). In a sequential testing strategy the TDx FLM assay performed well as the initial test. Our data indicate that the value for maturity suggested by the manufacturer may be more conservative than necessary. CONCLUSIONS The TDx FLM assay appears to be a useful test in the field of fetal lung maturity testing.