Objective: Our purpose was to determine whether a borderline amniotic fluid index observed during antepartum testing confers a significant risk of adverse perinatal outcome. Study Design: We conducted a retrospective review of all patients entering antepartum testing at Los Angeles County–University of Southern California Women’s and Children’s Hospital during a 4-month period beginning January 1, 1996. Women with singleton pregnancies who underwent antepartum testing within 1 week of delivery and who were delivered at our institution were identified for our study. An amniotic fluid index >5 and <10 cm was defined as “borderline” and an amniotic fluid index of 10 to 24 cm was considered normal. Markers of adverse perinatal outcome included intrapartum fetal distress, 5-minute Apgar score <7, meconium-stained amniotic fluid, and intrauterine growth restriction. Results: There was a 2-fold increase in the incidence of adverse perinatal outcome among the women with borderline amniotic fluid index in comparison with control subjects with normal amniotic fluid volume. This difference reflected a 4-fold increase in the incidence of fetal growth restriction among women with a borderline amniotic fluid index. Conclusions: A borderline amniotic fluid index observed in antepartum testing is associated with an increased risk of intrauterine growth restriction and overall adverse perinatal outcome. These observations suggest that borderline amniotic fluid index merits twice-weekly antepartum testing. (Am J Obstet Gynecol 1999;180:1461-3.)