Primary immunodeficiencies as well as autoimmune diseases have been associated to X chromosome abnormalities. Furthermore, the functional biology of the X chromosome is unique because genes located in this chromosome can undergo inactivation, and subsequently transcriptional silencing. Non-random X chromosome inactivation has been hypothesized to be involved in the development of autoimmunity. Recently X chromosome monosomy has also been proposed as a common etiologic mechanism for some autoimmune diseases. Herein, we review some of these findings above mentioned.