Leptin therapy improves insulin sensitivity in people with leptin-deficiency but it is not known whether it improves insulin action in persons who are not leptin deficient. The purpose of the present study was to determine whether leptin therapy has effects on insulin action in obese subjects with type 2 diabetes mellitus (T2DM). A randomized, placebo controlled trial was conducted in obese subjects with newly-diagnosed T2DM. Subjects were randomized to treatment with placebo, low-dose, or high-dose leptin. Insulin sensitivity was measured.