We studied the seroprevalence of antibodies to Chlamydia pneumoniae, the third species of chlamydia, in patients with acute infections in otolaryngeal sites. The patients were divided into five diagnostic groups, i.e. sinusitis, otitis media, tonsillitis, laryngitis and bronchitis group. Antibodies were measured by the microimmunofluorescent method. Antibodies confirming past infection with C. pneumoniae were found 46.2 per cent of the patients in the diagnostic groups and 44.7 per cent of the subjects in the control group. The difference between these two groups was not statistically significant. Acute or recent antibodies were detected in 10.5 per cent (2/19) of these patients with sinusitis, 19.2 per cent (10/52) of those with tonsillitis, 23.5 per cent (8/34) of those with otitis media, 18.2 percent (6/33) of those with laryngitis and 22.8 percent (13/57) of those with bronchitis, whereas only one member of the control group had acute antibodies. These data suggested that C. pneumoniae is an important and common pathogen of otolaryngeal disease.