In the last few years a great deal of information on the etiopathogenetic aspects of organ-specific autoimmune diseases (OSADS) has been obtained. It has been shown that genetic factors play an etiologic fundamental role. They are responsible for the dysregulation of the immune system and for the target organ susceptibility which favour the onset of the diseases. Putative environmental factors, such as viral infections, can act as initiating or precipitating events only in genetically predisposed individuals. Immunological mechanisms capable of triggering autoimmune responses have been demonstrated. Data obtained from experimental models and from humans suggest that the ongoing expansion of autoreactive T cells with specificity for autoantigens (AAgs) can be considered as the main immunological event capable of inducing and maintaining the target organ damage. These cells can activate different effector systems, i.e., autoantibody (AAb)-producing B cells, cells with cytotoxic activity, etc., by releasing different combinations of lymphokines. In overt diseases AAbs are directly involved in the pathogenesis of lesions due to autoimmune responses against functional molecules and cellular receptors. The pathogenesis of the common inflammatory destructive lesions of the target organs is more complex and not yet clarified. A large proportion of T cells present in the inflammatory infiltrates are apparently not directed to the AAgs. Most cells display cytolytic activity and may contribute to tissue damage by releasing lymphokines which activate other cells and cascade the process. Vicious cycles, i.e., upregulation of class II and I molecules, alterations of the cytokine network, etc., are supposed to be involved in the maintenance of target organ lesions.