The L-tryptophan eosinophilic myalgia syndrome (EMS) clinically has some similarities with idiopathic eosinophilic fasciitis (EF). In order to study the pathology of both syndromes, we analyzed 21 biopsies of patients with EMS and 8 with idiopathic EF. In both diseases there is dermal and fascial mucin and dermal edema, but this was more common in EMS. EMS is also characterized by dilated lymphatics, dermal and septal sclerosis and macrophage-rich inflammation. Neural inflammation was seen in 4 of the cases with EMS and in none with idiopathic EF. In both syndromes, there are many histopathological similarities. The differences may be due to sampling and to sample size. The nerve lesions of EMS may result from the nature of lymphocyte-macrophage inflammation, or the effect of the eosinophil neurotoxin and may not be a primary event.