Objective: Pregnancies complicated by diabetes are frequently characterized by an increased volume of amniotic fluid, and the pathophysiologic mechanism of this increase is not known. Our goal was to evaluate the relationship between amniotic fluid glucose concentration and the amniotic fluid index in pregnancies complicated by insulin-treated diabetes and to compare it with that seen in normal pregnancies. Study Design: Amniotic fluid index and amniotic fluid glucose levels were measured before elective repeated cesarean delivery in 41 women with insulin-treated diabetes and in 35 women without diabetes. Only singleton gestations without anomalous fetuses were included. Women with diabetes were hospitalized for approximately 4 weeks before delivery, during which time glycemic control was optimized. Amniotic fluid index and amniotic fluid glucose concentration were correlated with each other and were compared between the groups with and without diabetes. Results: The mean amniotic fluid index was significantly increased in the diabetes group (16.6 ± 5.0 cm in the diabetes group vs 13.4 ± 3.5 cm in the control group; P = .002). The amniotic fluid glucose concentration was also significantly greater in the diabetes group than in the control group (39 ± 17 mg/dL in the diabetes group vs 24 ± 11 mg/dL in the control group; P < .001). Among women with diabetes the amniotic fluid glucose concentration was significantly correlated with the amniotic fluid index (r = 0.32; P = .04), a correlation not found among the control women. The mean fasting blood glucose concentration among the women with diabetes for the week before amniocentesis was 82 ± 11 mg/dL. Conclusion: The amniotic fluid index parallels the amniotic fluid glucose level among women with diabetes. This finding raises the possibility that the hydramnios associated with diabetes is a result of increased amniotic fluid glucose concentration. (Am J Obstet Gynecol 2000;182:901-4.)