In 1981-1982, individuals in fourteen central and northwest provinces in Spain were affected by an illness that was eventually labeled toxic oil syndrome (TOS) by the World Health Organization. Thousands of individuals were diagnosed with, and 356 people eventually died from, the disease. The disease shares striking similarities with several autoimmune diseases, particularly eosinophilia-myalgia syndrome (EMS) and diffuse fasciitis with eosinophilia (DFE). As with many other autoimmune diseases, women were more severely affected than men and made up a significant portion of TOS-related deaths. While a number of etiologic agents were investigated, disease occurrence was found to be significantly associated with consumption of contaminated rapeseed oil produced by a particular refinery. Two compounds, 1,2-di-oleyl ester (DEPAP) and oleic anilide are considered to be biologically relevant contaminants that may contribute to disease development. Toxic oil syndrome was a three-phase disease with an initial non-necrotizing vasculitis in multiple organs. Suspected immune mechanisms in TOS include activation of T-cells, altered cytokine production, and several studies have associated disease severity with HLA-DR2 and polymorphisms in metabolism and immune response genes. While a number of animal models have been used to investigate the underlying immune mechanisms in TOS, only a few studies in rodents have demonstrated the classical symptoms of TOS. Biotransformation and oxidation of the parent compound(s) to reactive intermediates prior to induction of autoreactive pathways appears to be an important component of the disease process. These reactive intermediates could haptenate self-proteins and activate autoreactive T-cells, disrupt signal transduction, or induce apoptosis and necrosis to release abnormal forms of self-antigens. Although the TOS epidemic was limited to a discrete period of time, the origin of the contamination determined, and the spread of the disease halted by government intervention, the underlying immune mechanisms have yet to be elucidated.