The cause of primary pneumonia was diagnosed in 124 of 127 consecutive adult patients admitted to hospital with community-acquired illness. Pneumococcal infection was found in 96 (76%) patients and legionnaries' disease was the second commonest infection identified (15%). Other bacterial infections were uncommon. 11 patients had atypical pneumonia, including 7 with psittacosis. There were several mixed infections and most of the 11 patients with viral infections also had bacterial pneumonia. 19 patients died (15%) and mortality was associated with increasing age, the presence of coexisting disease, and the cause of the pneumonia. Recognition of the most likely causes of severe pneumonia allows logical initial antibiotic treatment for such patients admitted to hospital.