Thirty six patients admitted to the Infectious and Parasitic Diseases Ward of the Hospital das Clinicas, School of Medicine, University of Sao Paulo, were studied. Leptospiroses was considered as hypothetical diagnosis taking clinical, epidemiological and nonspecific laboratory features into account. Diagnosis was later confirmed using specific laboratory investigation, mainly the microscopic seroagglutination test, even through final diagnosis assessment could only be achieved several days after symptoms had started.