IFN-α exercises multiple immune modulatory and antiviral activities and has been suggested to play a critical role in the pathogenesis of systemic lupus erythematosus (SLE). Plasmacytoid dendritic cells (pDCs) release IFN-α upon TLR7 and TLR9 ligation. With respect to the nine times higher incidence of SLE in women and the clinical use of synthetic TLR ligands as novel immune adjuvants, we analyzed IFN-α and TNF-α production in healthy human individuals. Blood samples were incubated with synthetic TLR7 and TLR9 ligands. In three independent groups (n 1= 120, n 2= 101, and n 3= 123), analysis revealed a capacity of female PBLs to produce significantly higher IFN-α levels after TLR7 stimulation (p 1< 0.0000001, p 2< 0.0000001, and p 3< 0.0001) compared with male PBLs. In contrast, no sex differences were evident after TLR9 stimulation. TNF-α production after TLR7 stimulation and also total pDC numbers were not different between females and males. X-inactivation escape of the TLR7 gene was investigated in monoclonal B cell lines and, independently, in pDCs after cell sorting and single-cell picking, indicating regular silencing of one TLR7 allele in females. Additionally, exogenous 17β-estrogen and estrogen receptor antagonism did not indicate a significant role on TLR7-induced IFN-α production. Our data reveal for the first time a profound sex-dependent pathway of TLR7-induced IFN-α with higher production in females. These findings may explain the higher prevalence of SLE in females and the reported decreased therapeutic efficacy of synthetic TLR7 ligands in male individuals.