Abstract
This article describes the history of an outbreak of eosinophilia myalgia syndrome (EMS) in 1989, which was rapidly linked by researchers to the amino acid L-tryptophan (tryptophan), which is widely taken as a supplement. An alternative hypothesis was proposed for the cause of EMS, based on the safety associated with the use of tryptophan over decades and the possibility of contamination of the tryptophan during manufacture. The process by which this hypothesis has been substantiated and the contaminant identified is described here.