OBJECTIVE: This study was undertaken to compare the use of glyburide with insulin for the treatment of gestational diabetes mellitus (GDM) unresponsive to diet therapy. STUDY DESIGN: A retrospective study was performed among women with singleton pregnancies who had GDM diagnosed, with fasting plasma glucose 140 mg/dL or less on glucose tolerance testing, between 12 and 34 weeks who failed diet therapy from 1999 to 2002. We identified 584 women and compared those treated with insulin between 1999 and 2000 with women treated with glyburide between 2001 and 2002. Maternal and neonatal outcomes and complications were assessed. Statistical methods included univariate analyses and multivariable logistic regression. RESULTS: In 1999 through 2000, 268 women had GDM diagnosed and were treated with insulin; in 2001 through 2002, 316 women had GDM diagnosed of which 236 (75%) received glyburide. The 2 groups were similar with regard to age, nulliparity, and historical GDM risk factors; however, women in the insulin group had a higher mean body mass index (31.9 vs 30.6 kg/m 2 , P=.04), a greater proportion identified themselves as white (43%, 28%, P<.001) and fewer as Asian (24%, 37%, P=.001), and they had a significantly higher mean fasting on glucose tolerance test (105.4 vs 102.4 mg/dL , P=.005) compared with the glyburide group. There were no significant differences in birth weight (3599+/-650 g vs 3661+/-629 g, P=.3), macrosomia (24%, 25%, P=.7), or cesarean delivery (35%, 39 %, P=.4). Women in the glyburide group had a higher incidence of preeclampsia (12%, 6%, P=.02), and neonates in the glyburide group were more likely to receive phototherapy (9%, 5%, P<.05), and less likely to be admitted to the neonatal intensive care unit (NICU) (15%, 24%, P=.008) though they had a longer NICU length of stay (4.3+/-9.6 vs 8.0+/-10.1, P=.002). Posttreatment glycemic control data were available for 122 women treated with insulin and 137 women treated with glyburide. More women in the glyburide group achieved mean fasting and postprandial goals (86%, 63%, P<.001). These findings remained significant in logistic regression analysis. CONCLUSION: In a large managed care organization, glyburide was at least as effective as insulin in achieving glycemic control and similar birth weights in women with GDM who failed diet therapy. The increased risk of preeclampsia and phototherapy in the glyburide group warrant further study.