A pair of monozygotic twins discordant for systemic lupus erythematosus (SLE) were studied and no differences noted in their immune response to tetanus toxoid, keyhole lympet hemocyanin, DNCB, delayed sensitivity, or antibody titers to viruses. Both were noted to have biologically false positive serology at an early age, but only one twin developed SLE. The clinically unaffected twin underwent castration at an early age, suggesting that ovarian hormones may play an important role in the development of SLE.The various proposed etiologies of systemic lupus erythematosus (SLE) include viral infection (1). endocrine abnormality (2), genetic predisposition (3), and hypersensitivity to self with loss of immunologic