Expression of B cell-activating factor (BAFF), a critical B cell survival factor, is elevated in autoimmune and lymphoproliferative disorders. Mice overproducing BAFF develop systemic lupus erythematosus (SLE)-like disease and exhibit B cell activation of classical and alternative NF-κB-signaling pathways. We used a genetic approach and found that both NF-κB-signaling pathways contributed to disease development but act through distinct mechanisms. Whereas BAFF enhanced long-term B cell survival primarily through the alternative, but not the classical, NF-κB pathway, it promoted immunoglobulin class switching and generation of pathogenic antibodies through the classical pathway. Activation of the alternative NF-κB pathway resulted in integrin upregulation, thereby retaining autoreactive B cells in the splenic marginal zone, a compartment that contributes to their survival. Thus, both classical and alternative NF-κB signaling are important for development of lupus-like disease associated with BAFF overproduction. The same mechanisms may be involved in the pathogenesis of human SLE.