Obesity is often associated with low-grade inflammation and insulin resistance, which can lead to the development of type 2 diabetes. Now, Stolarczyk et al. report that mice lacking expression of the immune cell-specific transcriptional regulator T-bet are prone to obesity, but are also more sensitive to insulin than wild-type mice. Increased insulin sensitivity appeared early in life and depended on the adaptive immune system. Notably, the numbers of CD4+ and CD8+ T cells and the levels of several cytokines were decreased, whereas the frequency of FOXP3+ regulatory T cells was increased in the visceral (but not subcutaneous) adipose tissue depots of T-bet-deficient mice. Future studies will clarify the mechanisms through which T-bet expression in adaptive immune cells decouples obesity from insulin resistance.