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Polymyositis marked the clinical onset of Chagas's disease in a patient with rheumatoid arthritis. This is unusual, although clinically unimportant muscle involvement in trypanosomiasis has been described. The plasma cell infiltrate and vascular deposition of IgM and C3 suggest that the humoral immune system may play a role in the pathogenesis of chagasic polymyositis. It is not known whether the rheumatoid diseases predisposed to the polymyositis.