['We sought to determine whether, among women with gestational diabetes mellitus, referral to a telephonic nurse management program was associated with lower risk of macrosomia and increased postpartum glucose testing.', 'There was medical center-level variation in the percent of patients referred to a telephonic nurse management program at 12 Kaiser Permanente medical centers, allowing us to examine in a quasi-experimental design the associations between referral and outcomes.', 'Compared with women from centers where the annual proportion of referral nurse management was <30%, women who delivered from centers with an annual referral proportion >70% were less likely to have a macrosomic infant and more likely to have postpartum glucose testing (multiple-adjusted odds ratio, 0.75; 95% confidence interval, 0.57-0.98 and multiple-adjusted odds ratio, 22.96; 95% confidence interval, 2.56-3.42, respectively).'] Does referral to telephonic nurse management improve outcomes in women with gestational diabetes?