Through their functional diversification, distinct lineages of CD4(+) T cells can act to either drive or constrain immune-mediated pathology. Transcription factors are critical in the generation of cellular diversity, and negative regulators antagonistic to alternate fates often act in conjunction with positive regulators to stabilize lineage commitment(1). Genetic polymorphisms within a single locus encoding the transcription factor BACH2 are associated with numerous autoimmune and allergic diseases including asthma(2), Crohn's disease(3,4), coeliac disease(5), vitiligo(6), multiple sclerosis(7) and type 1 diabetes(8). Although these associations point to a shared mechanism underlying susceptibility to diverse immune-mediated diseases, a function for BACH2 in the maintenance of immune homeostasis has not been established. Here, by studying mice in which the Bach2 gene is disrupted, we define BACH2 as a broad regulator of immune activation that stabilizes immunoregulatory capacity while repressing the differentiation programs of multiple effector lineages in CD4(+) T cells. BACH2 was required for efficient formation of regulatory (T-reg) cells and consequently for suppression of lethal inflammation in a manner that was T-reg-cell-dependent. Assessment of the genome-wide function of BACH2, however, revealed that it represses genes associated with effector cell differentiation. Consequently, its absence during T-reg polarization resulted in inappropriate diversion to effector lineages. In addition, BACH2 constrained full effector differentiation within T(H)1, T(H)2 and T(H)17 cell lineages. These findings identify BACH2 as a key regulator of CD4(+) T-cell differentiation that prevents inflammatory disease by controlling the balance between tolerance and immunity.