The aim of this study was to develop methods for the measurement of sputum antibodies in the laboratory diagnosis of acute and chronic lower respiratory tract infections caused by Chlamydia pneumoniae. Paired serum specimens, sputum specimens, and pharyngeal or nasopharyngeal swabs were obtained from 97 patients; 51 of them had community-acquired pneumonia, and 46 had chronic obstructive pulmonary disease (COPD). C. pneumoniae-specific serum immunoglobulin G (IgG), IgA, and IgM antibodies were measured by the microimmunofluorescence (micro-IF) test. For sputa, specific IgA and IgG antibodies were measured by the micro-IF test and secretory IgA (sIgA) was measured by enzyme immune assay (EIA) with C. pneumoniae elementary bodies as the antigen. Sputum IgA and sIgA antibodies to C. pneumoniae were found, respectively, in 52 and 51% of the COPD patients. Elevated levels of stable serum IgG and IgA antibodies (IgG titer of > or = 128 and IgA titer of > or = 40), suggesting chronic infection, were found in 54% of the COPD patients. The sensitivity for the sputum IgA micro-IF test compared with elevated serum antibody levels was 87.5%, and that for the sputum sIgA EIA was 88%; the respective specificities were 90 and 95%. Acute C. pneumoniae infection was diagnosed in seven pneumonia patients, and two (29%) of these patients were positive by sputum EIA antibody measurements. Two pneumonia patients without acute infection had stable elevated IgG and IgA levels in their sera, and both of them were sputum antibody positive. We conclude that the measurement of IgA antibodies to C. pneumonia in sputum is a useful additional diagnostic tool for chronic C. pneumoniae infections.