This one year prospective multicentre study was designed to determine the incidence of community-acquired pneumonia in adults. It was carried out in primary health care centres and three reference hospitals, located in the 'Maresme' region (Barcelona, Spain) serving a population of 39,733 subjects over 13 years of age. Patients suspected of having contracted community-acquired pneumonia were visited by their family doctors and referred to the three reference hospitals for confirmation of the diagnosis. Patients attending the emergency services of these hospitals were also included. Urine and blood samples were obtained for culture, antigen detection, blood count, serological tests, blood gases and biochemical profile. The diagnosis of community-acquired pneumonia was made in 105 patients. Forty-six patients had an identifiable microbial etiology. Chlamydia pneumoniae was the most common pathogen (16 cases) followed by Streptococcus pneumoniae (13 cases) and Mycoplasma pneumoniae (8 cases). In conclusion; the annual incidence rate of community-acquired pneumonia in adults in this area was 2.6 cases per 1,000 inhabitants and Chlamydia pneumoniae was the most frequent causative pathogen.