Twenty cases of leptospirosis, (15 by Leptospira icterohaemorragiae, one by Leptospira canicola and four undetermined) were studied from a clinical, laboratorial and anatomo-pathological viewpoint (five necropsies and ten muscular biopsies). Important clinical elements for the differencial diagnosis of jaundice from other etiologies, were the sudden beginning with fever, intense muscular pain, congestion and conjunctival hemorrhage, besides the reddish color of the jaundice. In contrast, the liver function tests and the transaminasis showed little alteration. The most frequent hepatic lesion, were the trabecular disorganization and the atrophy of isolated hepatic cells which were more evident around the central lobular vein. The kidney has the aspect of colemic nephrosis; shows biliary impregnation of the cells, biliar cylinders on the tubules and predominant proximal tubular necrosis. An intense interstitial lymphocytic infiltration, binucleation, occasionally hemorrhagic infiltration and a serious degenerative pra- cess of the muscular fibers were observed. Congestion and punctiform hemorrhages predominated in the gasòro-intestinal tract, petechial hemorrhages of the pleura, edema and alveolar hemarrhages were seen in the lungs. Edema and fragmentation of the miocardiac fibers were constantly observed in the heart.