A retrospective study by means of indirect immunofluorescence was carried out to detect IgM antibodies against Chlamydia pneumoniae and Mycoplasma pneumoniae in seronegative patients for Hantavirus, with characteristics symptoms of atypical pneumonia. The final results indicate that the prevalence of C. pneumoniae reaches 8,6% and M. pneumoniae is equivalent to 17,1% of the patients studied. By this we can be aware of the importance of these agents in our media, and the necessity of having rapid laboratory techniques that allow an opportune differential diagnosis in pulmonary syndrome for Hantavirus and other pathologies that cause similar symptoms, mainly atypical pneumonia