Evidence from an array of scientific studies strongly supports the conclusion that ingestion of products containing L-tryptophan (LT) produced by Showa Denko KK caused the 1989 epidemic of eosinophilia-myalgia syndrome (EMS) in the United State. In case-control studies of EMS, LT exposure was essentially universal among cases but rare among controls. Of 6 manufacturers of LT, only LT manufactured by Showa Denko KK was clearly associated with illness. The data meet other Hill criteria for inferring a causal relationship. Consistent findings were found in multiple independently conducted studies. There was a dose-response effect, with risk of illness increasing as a function of the amount of tryptophan consumed. The extremely small p values observed in the multiple independently conducted studies effectively rule out the possibility that the tryptophan-EMS association was the result of chance. Moreover, no potential confounding factor or bias explains the association. The incidence of EMS in the United States diminished abruptly once LT containing products were recalled.