[Monozygotic twins with Klinefelter syndrome (concordant) and systemic lupus erythematodes (discordant)]

Monozygotic twins, both with Klinefelter's syndrome, are presented. One of them had clinical and serological evidence of systemic lupus erythematosus (SLE), whereas the other did not show any immunological disorder. In relation to this case of discordant SLE the role played by exogenous factors in triggering the manifestation of this autoimmune disease under a certain genetic predisposition is discussed.