SOCS1 profoundly influences the development and peripheral homeostasis of CD8+ T cells but has less impact on CD4+ T cells. Despite the moderate influence of SOCS1 in the development of the total CD4 T-cell lineage, we show here that SOCS1 deficiency resulted in a 10-fold increase in Foxp3(+) CD4(+) T cells in the thymus. Increased numbers of Foxp3+ thymocytes occurred in mice with T-cell-specific ablation of SOCS1, suggesting that the effect is T-cell intrinsic. This increase in Foxp3+ CD4+cells in SOCS1-deficient mice also occurred in the absence of IFN-gamma or/and IL-7 signaling. Increase in CD25+CD4+ T cells in the absence of SOCS1 could be partly due to enhanced survival by CD25+CD4+cells, to a lesser degree CD25-CD4+ T cells, from SOCS1-deficient mice with or without T-cell growth factors.