Background: The association between body mass index (BMI) and the development of type 2 diabetes (T2DM) is well established. The determinants of the resolution of T2DM with weight reduction after bariatric surgery, however, are not fully understood. Objective: Examine the course of T2DM during weight loss after bariatric surgery. Methods: Through a retrospective chart review, 56 morbidly obese, diabetic patients who underwent bariatric surgery were followed for 12 months. Resolution of T2DM was defined as an initial elevated glycohemoglobin (HbA1C) > 6.0% that decreased to < 6.0% with weight loss allowing discontinuation of all diabetic medication. Results: Of the 56 with T2DM, 31 (55.4%) resolved T2DM. The mean age ± SD was 49.0 ± 7.3 and 53.1 ± 7.0 years for resolved and persistent T2DM, respectively (p = 0.04). The baseline BMI was higher in the resolved group, measuring, 51.3 ± 8.3 kg/m2 compared with the persisted group of 46.5 ± 4.9 kg/m2, p = 0.02. The final BMI was 33.6 ± 7.6 kg/m2 and 33.9 ± 5.2 kg/m2, p = 0.87 for the resolved and persistent cases, respectively. The absolute and relative reductions in BMI were -17.6 ± 3.6 kg/m2 and -12.6 ± 6.3 kg/m2, p < 0.0001; and -34.7 ± 6.3% and -26.6 ± 11.9%, p = 0.002, over a mean duration of 12.9 ± 1.4 and 10.8 ± 5.2 months for resolved and persistent cases, respectively. Multiple logistic regression found factors such as a higher baseline BMI, p = 0.006, and lower final BMI, p = 0.03, were independent predictors of the resolution of T2DM while lower baseline HbA1C, p = 0.09, approached significance. There was a spike in HbA1C at 6 weeks in those with resolution of T2DM while there was a curvilinear decline in HbA1C in persistent T2DM with failure to normalize. Conclusions: Resolution of T2DM after bariatric surgery is a dynamic process and is associated with substantial weight loss and good glycemic control during the pre- and post-operative periods beyond 6 weeks.