Toxic oil syndrome or simply toxic syndrome (Spanish: síndrome del aceite tóxico or síndrome tóxico) is a musculoskeletal disease. A 1981 outbreak in Spain killed over 600 people and is thought to have been caused by contaminated colza oil. Its first appearance was as a lung disease, with unusual features: though the symptoms initially resembled a lung infection, antibiotics were ineffective. The disease appeared to be restricted to certain geographical localities, and several members of a family could be affected, even while their neighbours had no symptoms. Following the acute phase, a range of other chronic symptoms was apparent.
In the early 80s, one region in Spain experienced an epidemic of what seemed to be pneumonia, but with additional symptoms. It was eventually discovered to be a new disease, named toxic oil syndrome (TOS), because it is thought to be caused by consumption of adulterated olive oil. As a result of this disease, in the first two years over 20,000 people were affected and 356 people died. Those still living are at high risk of cardiovascular diseases. Scientists are still unsure of what exactly caused the disease, similar to an autoimmune disorder.