Objective: We evaluated whether improvements in pregnancy outcomes after treatment of mild gestational diabetes mellitus differed in magnitude on the basis of fetal gender. Study Design: This is a secondary analysis of a masked randomized controlled trial of treatment for mild gestational diabetes mellitus. The results included preeclampsia or gestational hypertension, birthweight, neonatal fat mass, and composite adverse outcomes for both neonate (preterm birth, small for gestational age, or neonatal intensive care unit admission) and mother (labor induction, cesarean delivery, preeclampsia, or gestational hypertension). After stratification according to fetal gender, the interaction of gender with treatment status was estimated for these outcomes. Results: Of the 469 pregnancies with male fetuses, 244 pregnancies were assigned randomly to treatment, and 225 pregnancies were assigned randomly to routine care. Of the 463 pregnancies with female fetuses, 233 pregnancies were assigned randomly to treatment, and 230 pregnancies were assigned randomly to routine care. The interaction of gender with treatment status was significant for fat mass (P =.04) and birthweight percentile (P =.02). Among women who were assigned to the treatment group, male offspring were significantly more likely to have both a lower birthweight percentile (50.7 ± 29.2 vs 62.5 ± 30.2 percentile; P <.0001) and less neonatal fat mass (487 ± 229.6 g vs 416.6 ± 172.8 g; P =.0005,) whereas these differences were not significant among female offspring. There was no interaction between fetal gender and treatment group with regard to other outcomes. Conclusion: The magnitude of the reduction of a newborn's birthweight percentile and neonatal fat mass that were related to the treatment of mild gestational diabetes mellitus appears greater for male neonates. © 2012 Mosby, Inc.