Ultrasound examination to measure the fetal biparietal diameter (BPD) and examine amniotic fluid for the presence or absence of free-floating particles was performed in 213 patients undergoing amniocenteses for maturity testing at term. A modified lecithin to sphingomyelin ratio (L/S) was performed on all samples. A BPD greater than or equal to 92 mm predicted 37 of 111 (33%) mature L/S ratios of nondiabetic pregnancies. The presence of free-floating particles predicted 48 of 138 (35%) mature L/S ratios of nondiabetics. Combined, a BPD greater than or equal to 92 mm and/or the presence of free-floating particles predicted 57 of 110 (52%) mature L/S ratios of nondiabetics. It is concluded that a BPD greater than or equal to 92 mm and the presence of freefloating particles are independent yet complimentary predictors of fetal lung maturity.