Establishing an early and certain diagnosis of leptospirosis has been a difficult task. So, we analysed the histopathological alterations of the gastrocnemius muscle, and studied, for the first time, the usefulness of the immune histochemical method of peroxidase-antiperoxidase for the demonstration of the spirochete and of its products in this tissue. Histopathological observations have shown a picture of myositis, characterized by interstitial inflammatory infiltrate and necrotic-degenerative abnormalities of muscle fibers. The lesions were considered minimal in 69.45% of the patients, moderate in 19.45%, severe in 5.55% and absent in the remaining. On the other hand, the immune histochemical method identified the etiology in 94.45% what was considered very expressive.