In 1981, in Spain, the ingestion of an oil fraudulently sold as olive oil caused an outbreak of a previously unrecorded condition, later known as toxic oil syndrome (TOS), clinically characterized by intense incapacitating myalgias, marked peripheral eosinophilia, and pulmonary infiltrates. Of the 20,000 persons affected, approximately 300 died shortly after the onset of the disease and a larger number developed chronic disease. For more than 15 years, a scientific committee supported by the World Health Organization's Regional Office for Europe and by the Institute of Health Carlos III in Madrid has guided investigation intended to identify the causal agent(s), to assess toxicity and mode of action, to establish the pathogenesis of the disease, and to detect late consequences. This report summarizes advances in research on this front. No late mortality excess has been detected. Among survivors, the prevalence of some chronic conditions (e.g., sclerodermia, neurologic changes) is high. Attempts to reproduce the condition in laboratory animals have been unsuccessful, and no condition similar to TOS has been reported in the scientific literature. Laboratory findings suggest an autoimmune mechanism for TOS, such as high levels of seric soluble interleukin-2 receptor. Epidemiologic studies integrated with chemical analyses of case-related oils have shown that the disease is strongly associated with the consumption of oils containing fatty acid esters of 3-(N-phenylamino)-1,2-propanediol (PAP). These chemicals have also been found in oils synthesized under conditions simulating those hypothesized to have occurred when the toxic oil was produced in 1981. Whether PAP esters are simply markers of toxicity of oils or have the capability to induce the disease remains to be elucidated.