The role of Chlamydia pneumoniae in respiratory infections is controversial. Seroepidemiological studies in different countries have shown an incidence of around 50% of previous contact, whereas prospective studies only in the USA have detected 6-10% of C. pneumoniae in community acquired pneumonias; in a similar Swedish study no cases were detected. There have been no descriptions of this Chlamydia in Mediterranean countries. The acute antibody response indicative of recent infection is accepted if the microimmunofluorescence test shows a fourfold rise in antibody or a titer 1/6 or higher of the IgM fraction. Since the introduction of the microimmunofluorescence test in our hospital, we have detected 4 cases of pneumonia associated to TWAR antibody in a current study of 63 cases of community acquired pneumonias which required hospitalization. All 4 patients presented chronic cardiopulmonary diseases. These facts and additional seroepidemiological studies with a frequency of 53% of previous contact, show that C. pneumoniae has a nonrare etiology in Spain and could be an endemic condition.