This study assessed whether a weight-gain restriction regimen, with or without exercise, would impact glycemic control, pregnancy outcome, and total pregnancy weight gain in obese subjects with gestational diabetes mellitus (GDM). A total of 96 subjects with GDM met the inclusion criteria and were sequentially recruited, with 39 subjects self-enrolled in the exercise and diet (ED) group, and the remaining 57 subjects self-enrolled in the diet (D) group owing to contraindications or a lack of personal preference to exercise. All patients were provided a eucaloric or hypocaloric consistent carbohydrate meal plan and instructed in the self-monitoring of blood glucose. In addition, all ED subjects were prescribed an exercise routine equivalent to a 60% symptom-limited VO2 max. Subjects were followed at weekly or biweekly office visits. Results showed maternal weight and body mass index (35.2+/-7.2 (ED) vs. 33.5+/-9.2 (D)) at study entry as well as number of weeks into the study (7.7+/-5.7 (ED) vs. 9.4+/-4.7 (D)) were similar in both the ED and D groups. Weight gain per week was significantly lower in the ED group than in the D group (0.1+/-0.4 kg vs. 0.3+/-0.4 kg; p<0.05). Subjects (either ED or D) who gained weight had a higher percentage of macrosomic infants than those subjects who lost weight or had no weight change during pregnancy. Other pregnancy and fetal outcomes such as complications, gestational age at delivery, and rate of cesarean delivery were similar in both groups. Conclusions of this study were that caloric restriction and exercise result in limited weight gain in obese subjects with GDM, less macrosomic neonates, and no adverse pregnancy outcomes. Pregnancy is an ideal time for behaviour modification, and this intervention may also help promote long-term healthy lifestyle changes.