The neuromuscular pathology of the Eosinophilia-Myalgia syndrome. The Eosinophilia-Myalgia Syndrome (EMS) is a recently reorganized disorder in patients ingesting pharmacologic doses of L-tryptophan. We studied the lesions of skeletal muscle, peripheral nerve and skin in 12 cases of EMS. Perimyositis was severe in four, moderate in two, mild in three and absent in three cases. The lesions contained many eosinophils, T-helper cells, mast cells and activated macrophages. Type 2 myofiber atrophy was present in five cases and in one, this was the only pathologic finding. Severe epineurial inflammation was seen in the three sural nerve biopsies. Indirect evidence for peripheral neurologic involvement in three other cases consisted of inflammation surrounding intramuscular nerve twigs (two cases) and neurogenic atrophy (one case). Phlebitis accompanied the connective tissue inflammation in five cases and endarteritis in one. Fasciitis was present in three of four skin biopsies and dermal fibrosis in one. 