OBJECTIVE: To pilot, among women with gestational diabetes mellitus (GDM), the feasibility of a prenatal/postpartum intervention to modify diet and physical activity similar to the Diabetes Prevention Program. The intervention was delivered by telephone, and support for breastfeeding was addressed. RESEARCH DESIGN AND METHODS: The goal was to help women return to their prepregnancy weight, if it was normal, or achieve a 5% reduction from prepregnancy weight if overweight. Eligible participants were identified shortly after a GDM diagnosis; 83.8% consented to be randomly assigned to intervention or usual medical care (96 and 101 women, respectively). The retention was 85.2% at 12 months postpartum. RESULTS: The proportion of women who reached the postpartum weight goal was higher, although not statistically significant, in the intervention condition than among usual care (37.5 vs. 21.4%, absolute difference 16.1%, P=0.07). The intervention was more effective among women who did not exceed the recommended gestational weight gain (difference in the proportion of women meeting the weight goals: 22.5%, P=0.04). The intervention condition decreased dietary fat intake more than the usual care (condition difference in the mean change in percent of calories from fat: -3.6%, P=0.002) and increased breastfeeding, although not significantly (condition difference in proportion: 15.0%, P=0.09). No differences in postpartum physical activity were observed between conditions. CONCLUSIONS: This study suggests that a lifestyle intervention that starts during pregnancy and continues postpartum is feasible and may prevent pregnancy weight retention and help overweight women lose weight. Strategies to help postpartum women overcome barriers to increasing physical activity are needed.