Study of 18 biopsy specimens in 11 patients with L-tryptophan-related eosinophiliamyalgia (fasciitis) syndrome showed hyaline sclerodermoid changes. Dermal scleroderma was found in eight of nine punch biopsy specimens and eight of nine excisional biopsy specimens. Fascial scleroderma was found in eight excisional biopsy specimens. One specimen obtained by excision had no fascia present. Eleven biopsy specimens showed edema of the dermis, and 13 showed dilated lymphatic structures; thus, the clinical picture of edematous sclerosis was confirmed. Mucinous fasciitis was present in five excisional biopsy specimens, in conjunction with a large number of macrophages in four. Dermal mucinosis was present in 11 biopsy specimens. Lymphocytic and macrophage inflammation was minimal in 14 biopsy specimens and pronounced in only 4. Plasma cells were present in eight cases. Eosinophils were present in substantial numbers in three biopsy specimens and only occasionally in four. Eosinophilic spongiosis was observed in one patient. Lymphocytic inflammation was noted around a single muscle spindle and around large nerve trunks in three patients. No relationship was established between these pathologic features and the duration or dose of tryptophan, prednisone treatment, or duration of symptoms. Pathologic features of the L-tryptophan syndrome consist of hyaline sclerodermoid collagen in the dermis, the septa, and the fascia. Edema, focal mucinosis, and macrophage inflammation may be features that identify this event.