A 1981 outbreak in Spain which affected about 25,600 people, with over 4,000 dying within a few months and a few thousand remaining disabled, is thought to have been caused by contaminated colza (rapeseed) oil.

In 1991, Posada de la Paz et al. found that French rapeseed oil was imported to Spain after denaturation with 2% aniline, allegedly for industrial use.

[3] None of the in vivo or in vitro studies performed with toxic-oil-specific components, such as fatty acid anilides, and esters of PAP, have provided evidence that these markers are causally involved in the pathogenesis of TOS.

[3] Data discrepancies combined with both a high level of secrecy surrounding the huge investigation and the fact that the first cases of the syndrome were located in Madrid (near the U.S. military base in Torrejón de Ardoz) spread the idea of a conspiracy.

Another theory suggests the toxic reaction was triggered by organophosphate poisoning (e. g., from pesticide residues in tomatoes) and covered up by the Spanish government and the WHO.