Autoimmune diseases are some of the most common yet least understood maladies in medicine today. Some estimates place the number of sufferers of such diseases as high as 20% of the U.S. population, most of them women. The disorders involved range from the familiar to the relatively uncommon to the obscure. What these disorders have in common is that they cause the immune system to attack the body's own tissues. Uncertainty in the field of autoimmune disease extends even to the fundamental questions of what an autoimmune disease is and how many people are affected; identifying specific environmental risk factors for autoimmune diseases is still highly speculative. To gain some sense of direction, scientists are looking at a few documented environmental exposures that have led to autoimmune syndromes, as well as some animal models that exhibit autoimmune syndromes similar to those seen in humans. At a September 1998 NIEHS workshop on environmental links to autoimmune diseases, participants prioritized research needs in five main categories, a step that should help scientists develop strategies for investigating this family of diseases.