Regulatory T cells (Tregs) are a subset of T lymphocytes that are responsible for suppressing the function of other immune cells, and preventing potentially harmful autoimmune responses. Studies in autoimmune-prone mice and human autoimmune diseases have shown reduced Treg number or function as a causative factor for the apparent loss of tolerance that contributes to disease. We have found that Lyn-deficient mice, which develop high titers of autoantibodies with age, have a perturbed Treg compartment. Contrary to what has been observed in some strains of autoimmune-prone mice, aged Lyn-deficient mice have increased numbers of Tregs. This expansion occurs in the presence of elevated serum IL-2 and diminished TGF-?. Despite expansion of the Treg compartment, Lyn-deficient mice succumb at 1 year of age due to immune complex-mediated glomerulonephritis. We have shown that Lyn is not expressed in Tregs or indeed in any T cell subset, suggesting that the expansion and apparent functional deficiency in Tregs in Lyn-deficient mice is due to extrinsic factors rather than an intrinsic Treg defect. Indeed, using an in vivo colitis model, we have shown that Lyn-deficient Tregs can suppress inflammation. These results suggest that Tregs are expanding in Lyn-deficient mice in an effort to control the autoimmune disease but are simply overwhelmed by the disease process. This study highlights the role of the inflammatory setting in autoimmune disease and its consideration when contemplating the use of Tregs as an autoimmune therapy.