Some helminths are major human pathogens. Recently, however, increased understanding of the immunoregulatory responses induced by this class of parasites, in combination with epidemiologic and animal studies, suggests that helminths may have therapeutic potential in autoimmune diseases (AD) and other conditions. This article reviews the rationale for and results of clinical trials to test the safety and efficacy of helminth therapy in AD. Also discussed are future prospects for investigation and the possibility that helminth treatment may serve as a probe to help reveal the pathogenesis of AD.