The participation of eosinophils in the Spanish toxic oil syndrome (TOS) was investigated. Eosinophil infiltration and degranulation in tissues from 52 patients with the TOS were examined by immunofluorescence staining for the eosinophil granule major basic protein (MBP). Serum MBP levels were determined in sera from 323 patients. Eosinophil infiltration and degranulation were found in several tissues, especially during the acute phase of the TOS, and serum MBP was significantly elevated during all phases of the disease, suggesting that eosinophils play a role in the pathogenesis of the TOS.