To determine the immunopathogenesis of the persistent symptoms of patients with eosinophilia-myalgia syndrome (EMS) induced by L-tryptophan, we performed immunocytochemical studies on 10 muscle and fascia biopsy specimens obtained during the acute disease and the chronic persistent connective tissue sclerosis. A series of monoclonal antibodies was used in a single- or double-immunostaining technique to detect and quantify T-cell subsets, macrophages, major histocompatibility complex antigens, eosinophilic basic protein-positive cells, and resting fibroblasts expressing Thy-I antigen or activated fibroblasts expressing the activation marker F-19. We found inflammatory cells consisting of CD8+ cells (45% +/- 8.9%), T4 cells (36% +/- 10.1%), and macrophages (19% +/- 12%), scattered or perivascularly in the fascia, the perimysium, and the endomysial septae. Only rare granulated or degranulating eosinophils were noted. Many muscle fibers around fascicles or near blood vessels expressed major histocompatibility complex-I antigens. The mean number of fibroblasts in the fascia, the perimysial connective tissue, and the spindle capsule was increased in the EMS patients' specimens compared with the endomysial cells seen in six disease-control muscle biopsy specimens from patients with chronic inflammatory myopathies or dystrophies (P < .01). Up to 70% of the fibroblasts in EMS were activated and up to 30% of them expressed HLA-DR antigen. In the disease controls up to 29% of the fibroblasts were activated but none expressed DR. Repeat muscle biopsy a year later in a patient whose symptoms persisted showed reduced inflammation but an increased number of activated fibroblasts and enhanced DR expression. We conclude that in EMS there is a T-cell-mediated process against components of the extracellular matrix, including fibroblasts, in the fascia and the perimysium that persists even years after the drug is discontinued. Because the fibroblasts are activated and aberrantly express DR antigen, they may be the target cells playing a role in the continuing clinical and histologic signs of tissue sclerosis.