TLR9 suppresses TLR7-driven pathogenesis in the MRL.Fas(lpr) murine model of systemic lupus erythematosus, but the mechanisms by which TLR7 promotes and TLR9 prevents disease in this and other lupus models remain unclear. Type I IFNs (IFN-I) have also been implicated in the pathogenesis of lupus both in patients and in several murine models of disease, but their role in MRL.Fas(lpr) mice is controversial. Using MRL.Fas(lpr) mice genetically deficient in a subunit of the receptor for IFN-I, Ifnar1, we show that IFN-I contribute significantly to renal disease in this model. Ifnar1 had no effect on anti-nucleosome or anti-Sm autoantibody titers, but instead regulated anticytoplasmic and anti-RNA specificities. Moreover, Ifnar1 deficiency prevented the exacerbation of clinical disease observed in Tlr9-deficient animals in this lupus model. Thus, IFN-I signaling is an important mediator of lupus pathogenesis and anti-RNA Ab production that is dysregulated in the absence of Tlr9.