The fluorescence polarization of amniotic fluid, a measure of fetal lung maturity, was determined in 518 amniotic fluid samples obtained by amniocentesis. The subjects were divided into seven clinical groups: premature contractions, premature rupture of the membranes, pregnancy-induced hypertension, diabetes, intrauterine growth retardation, vaginal bleeding, and “other” for gestational-age groups of 30 weeks or less, 31-36 weeks, and 37 or more weeks. The proportion of mature values (fluorescence polarization 0.285 or lower) increased progressively from 12.5% at 27-28 weeks to 100% at 39-40 weeks. In the 31-36-week gestation group, the proportion of mature values in subjects with premature rupture of the membranes (84.6%) was significantly higher than in those with premature contractions (60%), severe pregnancy-induced hypertension (50%), mild pregnancyinduced hypertension (55.2%), diabetes class A (50%), insulin- dependent diabetes (60%), and other (63.5%). The mean ± SD fluorescence polarization value was significantly lower in premature rupture of the membranes (0.256 ± 0.030) than in premature contractions (0.274 ± 0.032), mild and severe pregnancy-induced hypertension (0.280 ± 0.027 and 0.280 ± 0.035, respectively), and class A and insulindependent diabetes (0.285 ± 0.018 and 0.277 ± 0.030, respectively). The severity of pregnancy-induced hypertension and diabetes did not appear to influence either the fluorescence polarization value or the proportion of mature results. With the exception of a marked influence of premature rupture of the membranes, abnormal pregnancy conditions did not appear to have a significant effect on fluorescence polarization of amniotic fluid.