Granzyme B is a unique serine protease, which plays a crucial role for target cell death. Several mechanisms of delivery of granzyme B to target cells have been recently identified. Granzyme B directly activates Bid, a specific substrate for granzyme B, resulting in caspase activation. Granzyme B efficiently cleaves many prominent autoantigens, and the hypothesis that autoantibodies arise when cryptic determinants are revealed to the immune system has been proposed. Some autoantibodies directed against granzyme B-specific neoepitopes are present in serum from patients with autoimmune diseases. In the tissues from autoimmune diseases, granzyme B might play an important role for disease progression (i.e., rheumatoid arthritis synovium) or inhibition (i.e., regulatory T cells). We have identified a novel type of activation-induced cell death (granzyme B leakage-induced cell death). Activation-induced natural killer (NK) cell death is accompanied by the leakage of granzyme B from intracellular granules into the cytoplasm, and it triggers apoptosis by directing Bid to mitochondrial membranes. An excess of "leaked" granzyme B over its inhibitor, serpin proteinase inhibitor 9, is a major determinant of cell death. The role of granzyme B in autoimmunity and its influence on NK cell death are discussed.