Estrogens promote lupus in humans and some mouse models of this disease. Nonetheless, little is known about the role of estrogen receptors in lupus pathogenesis. Here, we report that in females on the lupus-prone (NZB× NZW) F 1 background, disruption of estrogen receptor-α (ERα or Esr1) attenuated glomerulonephritis and increased survival. ERα deficiency also retarded development of anti-histone/DNA antibodies, suggesting that ERα promotes loss of immunologic tolerance. Furthermore, ERα deficiency in (NZB× NZW) F 1 females attenuated the subsequent development of anti-double-stranded DNA (dsDNA) IgG antibodies, which are associated with glomerulonephritis in this model. We provide evidence that ERα may promote lupus, at least in part, by inducing interferon-γ, an estrogen-regulated cytokine that impacts this disease. ERα deficiency in (NZB× NZW) F 1 males increased survival and reduced anti-dsDNA antibodies, suggesting that ERα also modulates lupus in males. These studies demonstrate that ERα, rather than ERβ, plays a major role in regulating autoimmunity in (NZB× NZW) F 1 mice. Furthermore, our results suggest for the first time that ERα promotes lupus, at least in part, by impacting the initial loss of tolerance. These data suggest that targeted therapy disrupting ERα, most likely within the immune system, may be effective in the prevention and/or treatment of lupus.