The eosinophilia-myalgia syndrome associated with the use of oral preparations of the amino acid L-tryptophan was recognized in late 1989. We describe the clinical and laboratory manifestations, pathological findings and early clinical course of 20 patients with the eosinophilia-myalgia syndrome. Prominent clinical findings included severe myalgias limiting function, fatigue, rashes, edema and weight gain, weight loss, muscle weakness and shortness of breath. Laboratory findings included eosinophilia (often marked), normal erythrocyte sedimentation rate, and elevated aldolase with normal or low creatine kinase values. On biopsy fascial inflammation was always seen consisting of lymphocytes, histiocytes and eosinophils in a perivascular distribution. Invasion of the vascular wall by lymphocytes was seen in 20%. Capillary and arteriolar endothelial cell thickening was found in most cases on electron microscopy and endothelial cell necrosis or mural invasion by lymphocytes was seen in 25% of cases. Two patients improved with no therapy. Ten patients responded to therapy with prednisone alone. Three patients have had progressive disease and one of these died. The relationship of this syndrome to previously described disease entities associated with eosinophilia is discussed.