Pathologic findings in eight patients with the eosinophilia-myalgia syndrome, secondary to L-tryptophan ingestion, are reported. Tissue was obtained by biopsy alone in six patients, by biopsy and autopsy in the seventh patient, and by autopsy alone in the eighth patient. Muscle biopsies in five patients demonstrated an inflammatory infiltrate composed predominantly of lymphocytes, histiocytes, plasma cells, and a few eosinophils. The inflammation involved the perimysial and epimysial connective tissue, the walls of some small blood vessels, the perineurium of small nerve twigs, muscle spindles, and fibrous septae of subcutaneous adipose tissue. In two patients with peripheral neuropathy and one patient without overt neuropathy, denervation atrophy of muscle and perimysial and epimysial fibrosis were present. Sural nerve biopsy tissue taken from two patients displayed prominent axonopathy in one, and minimal changes in the second. Pulmonary changes in the two autopsied patients included endothelial cell damage, endovasculitis and fibromyxoid intimal change im arteries and veins, and interstitial pneumonitis with fibrosis.