The authors present the results of a histopathologic study in muscle biosy of 16 cases of leptospirosis. The most important findings were focal necrosis of the muscle fibers, hemorrhagic infiltration in the necrotic focus and interstitial hemorrhages; the inflamatory reaction was very rare or almost inexistent. Besides those findings attention was called to sacoplasmatic vacuolization of muscle fibers preceeding the muscle necrosis. The multinuclear sacoplasmatic strueture seems to be an attempt at muscle regeneration, well succeeded. Finally, the authors consider that muscle alterations, of not generally not specific of leptospirosis, are very characteristic and help to diagnose the disease.