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To determine the etiology of community-acquired pneumonia in the adult population of a defined area, specific antibody responses in paired serum samples, levels of circulating pneumococcal immune complexes in serum samples, and pneumococcal antigen in urine were measured. Samples (304 paired serum samples and 300 acute urine samples) were obtained from 345 patients > or =15 years old with community-acquired, radiologically confirmed pneumonia, which comprised all cases in the population of 4 municipalities in eastern Finland during 1 year. Specific infecting organisms were identified in 183 patients (including 49 with mixed infection), as follows: Streptococcus pneumoniae, 125 patients; Haemophilus influenzae, 12; Moraxella catarrhalis, 8; chlamydiae, 37 (of which, Chlamydia pneumoniae, 30); Mycoplasma pneumoniae, 30; and virus species, 27. The proportion of patients with pneumococcal infections increased and of those with Mycoplasma infections decreased with age, but for each age group, the etiologic profile was similar among inpatients and among outpatients. S. pneumoniae was the most important etiologic agent. The annual incidence of pneumococcal pneumonia per 1000 inhabitants aged > or =60 years was 8.0.

Between September 1, 1981, and August 31, 1982, all patients with suspected or confirmed pneumonia among the 46,979 inhabitants of four municipalities in the province of Kuopio, Finland, were reported to a pneumonia register by their attending physicians. In addition, two study pathologists reported all cases of pneumonia found at autopsy, and two permanent registers were checked for retrospective identification of patients. Chest radiographs were obtained from 97% of all patients. The final diagnosis was based on radiologic or autopsy criteria. A total 546 patients (323 males and 223 females) had community-acquired pneumonia; of these, 37% were less than 15 years of age, and 31% were 60 years of age or older. Nineteen percent of the patients had defined chronic conditions, and 42% were admitted to hospital. The case fatality rate was 4%. The overall incidence of community-acquired pneumonia per 1,000 inhabitants per year was 11.6 (13.9 in males, 9.4 in females). The age-specific incidence per 1,000 inhabitants per year was as follows: age < 5 years, 36.0; age 5-14 years, 16.2; age 15-59 years, 6.0; age 60-74 years, 15.4; and age > or = 75 years, 34.2.