NK cells are a subset of mononuclear cells which have long been suspected of playing an immunoregulatory role in the prevention of autoimmune diseases. Here, we briefly discuss the characteristics of NK cells--particularly what is known of their functional capabilities--and summarise the major findings from studies of NK cells in human and animals susceptible to three major autoimmune diseases: multiple sclerosis, systemic lupus erythematosus and type 1 (autoimmune) diabetes mellitus. In each case, we present the evidence for an association between disease and deficiencies in NK cells. The prospect of clinical interventions that stimulate NK cell activity are discussed and the current status described.