Large-for-gestational-age (LGA) fetuses, especially those whose birth weight is greater than 4000 g, are at elevated risk for obstetrical complications. Prenatal diagnosis of the condition has been limited by the inaccuracy of fetal weight estimation and by low positive predictive values of other sonographic parameters. We studied 412 fetuses, 35 LGA and 377 non-LGA, who were scanned within 1 week of delivery, to determine if assessment of amniotic fluid volume can be used to improve the accuracy of fetal weight estimation for diagnosing or excluding LGA. In our study, the positive and negative predictive values of fetal weight estimation alone were 47% and 95%, respectively. The distribution of amniotic fluid volumes in LGA and non-LGA fetuses differed significantly (P less than .001). Polyhydramnios occurred more frequently in LGA fetuses than non-LGA fetuses (17% vs 8%) but had little effect on the positive predictive value of a fetal weight estimate above the 90th percentile. Oligohydramnios occurred less frequently in LGA than non-LGA fetuses (3% vs 19%) and improved the negative predictive value of fetal weight estimation. In particular, the combination of oligohydramnios and a fetal weight estimate below the 90th percentile virtually excluded the possibility of LGA.