Leptospirosis is a worldwide zoonosis, caused by Leptospira interrogans. At the earlier stage of the disease, the IgM immunoassays are expected to have more sensitivity than other immunoassays. Previous reports showed that the indirect immunofluorescent assay for detection of immunoglobulins (IFA-Igs) against Leptospira spp showed higher sensitive and specific than some genus specific tests. The authors determined an efficacy of the indirect immunofluorescent assay for detection of IgM specific leptospiral antibody (IFA-IgM). One hundred and eighty patients with acute febrile illness without localizing signs admitted to Hat Yai Hospital were studied. Using the cut-off value of > or = 1:400, the overall sensitivity, and overall specificity of the IFA-IgM were 89.2 per cent and 95.1 per cent, respectively. They were slightly greater than those of the conventional IFA-Igs (86.5% and 91.6%). The first sera obtained from patients including acute sera and single sera showed a low sensitivity (32.4%) but still higher than the IFA-Igs (29.7%). None of the patients with various diseases commonly confused with leptospirosis or healthy blood donors gave a titer greater than 1:200. In conclusion, the IFA-IgM has the same protocol as the IFA-Igs but the sensitivity and specificity is slightly greater than the IFA-Igs. This is another alternative test for the diagnosis of acute leptospirosis.