The aetiology of community-acquired pneumonia was studied by use of new bacterial and established viral serological methods besides blood culture in 162 patients. Evidence for a specific aetiology was obtained in 79 patients (49.4%). The pneumococcus was the most common aetiological agent, identified in 25.6% of cases. Other bacteria, Haemophilus influenzae, Branhamella catarrhalis, Neisseria meningitidis and Chlamydia spp. were demonstrated in 23.5%, Mycoplasma pneumonia in 1.2% and viruses in 7.4% patients. In 58% those with viral pneumonia there was evidence of mixed infection with bacteria. The predictive value of rapid laboratory tests, erythrocyte sedimentation rate, white blood cell count and C-reactive protein (CRP), was evaluated in relation to the aetiological diagnosis. They all differentiated viral from bacterial pneumonia, with CRP having the best predictive value. On the basis of these tests, most cases in which our serological tests remained negative would appear to have a bacterial aetiology also.