A year long multicentre prospective study was carried out in the Valencia region of Spain, to determine the cause of community acquired pneumonia. The study was based on 510 of 833 patients with pneumonia. Of these, 462 were admitted to hospital, where 31 patients died. A cause was established in only 281 cases--208 of bacterial, 60 of viral, and 13 of mixed infection. The most common microorganisms were Streptococcus pneumoniae (14.5%), Legionella sp (14%), Influenza virus (8%), and Mycoplasma pneumoniae (4%). There was a higher incidence of Legionella sp than in other studies.