Thirty-seven sera samples from patients with leptospirosis icterohaemorrhagic form were studied with a time interval of 2 to 12 days between the beginning of the symptoms and the collection blood samples. It was isolated leptospira of 5 patients' heinocultures (13.5%) and from 4 of these the etiological agent pertained to the serogroup Icterohaemorrhagiae serovar copenhageni. Thirty-five of them (94.6%), including the four patients whose the etiological agent was isolated, showed reactivity in the enzyme linked immunosorbent (ELISA) IgM test. By this way, it was demonstrated that this test is important for a rapid diagnosis of human leptospirosis, even in the beginning of the disease, when there is still leptospiraemic phase.