This study was undertaken to determine the course of unconjugated plasma estriol (E3) concentrations after 40 weeks' gestation and to assess whether the latter may be used as a first-line test in the management of term pregnancies with poor dates and prolonged gestation. Plasma E3 was measured twice weekly in 134 women with well-dated pregnancies of 40 weeks or more and in 213 women with pregnancies of unverified age but believed to be in excess of 40 weeks from the last menstrual period. The obstetric management was based upon twice weekly determinations of urinary E3 and antepartum heart rate testing, but not all patients complied. Plasma E3 levels were not availabe for management of the patients. The results indicate that unconjugated plasma E3 levels are highest at 40 weeks' gestation and decline thereafter by 12% each week. Because of this slow decline, biweekly assays suffice. Increasing and plateauing serial plasma E3 levels reveal that a patient approaches term or has reached 39 to 41 weeks' gestation, respectively, whereas decreasing E3 concentrations indicate that the patient is well beyond 40 weeks, and that her fetus may be about to or has become postmature. Only seven of the 347 patients studied were delivered of a postmature infant. Their last plasma E3 levels ranged from 3.8 to 8.0 ng/ml, i.e., were below the 95% confidence limits established for normal term pregnancies. It was found that 73% of the 347 patients had plasma E3 levels of 12 ng/ml or more, and that all their pregnancies ended normally, except for three instances of presumed intrapartum fetal distress. However, in 19 (20%) of the other 27% of the 347 patients who had plasma E2 levels of less than 12 ng/ml, abnormal outcome was encountered: postmaturity (seven), fetal death (four), intrauterine growth retardation (three), malformations (two), or intrapartum fetal distress (three). These results allow the conclusion that twice weekly assays of unconjugated plasma E2 may be used as a first-line test in the management of prolonged gestation.