A total of 2702 transabdominal amniocenteses performed at the Los Angeles County--University of Southern California Medical Center were reviewed, with particular emphasis on 392 samples performed beyong 41 weeks' gestation. A significant rise in the percent of amniocenteses with meconium staining was found to occur at an beyond 39 weeks. Meconium-stained fluid at amniocentesis was found to be associated with an increased incidence of babies weighing greater than 4000 g, maternal diabetes mellitus, and cesarean deliveries, in comparison to samples with clear amniotic fluid. Infants with meconium-stained fluid had an increased incidence of low 1-minute Apgar scores, but all 5-minute Apgar scores were 7 or greater. Ten perinatal deaths occurred after an amniocentesis with clear fluid in prolonged pregnancy, with four of these occurring within 7 days of amniocentesis. Lecithin/sphingomyelin (L/S) ratios less than 2.0 were found in 6% of amniocenteses performed beyond 41 weeks. However, none of the newborns with low L/S ratios develop subsequent neonatal respiratory distress syndrome. Amniotic fluid creatine values or blood-contaminated fluid were not found to be correlated with fetal outcome. No fetal mortality was attributable to amniocentesis. In view of the significant amount of false-positive and false-negative results, and the rare inherent danger associated with amniocentesis, its use solely to demonstrate the presence or absence of meconium staining appears to be of questionable value in the management of prolonged pregnancy.