Community-acquired adult lower-respiratory-tract infections (LRTI) are generally thought to be caused by atypical and viral infections. We have studied 480 adults presenting to a single general practice with community-acquired LRTI between November, 1990, and December, 1991. The overall incidence was 44 cases per 1000 population per year; the incidence was 2-4 times higher in people aged 60 and over than in those aged less than 50. 206 patients were studied in detail; among this group 91 (44%) had 113 pathogens identified. There were 92 bacteria (Streptococcus pneumoniae in 62 and Haemophilus influenzae in 16), 19 viruses (influenza virus in 12), and only 2 atypical pathogens (Mycoplasma pneumoniae and Coxiella burnetii). Pneumococcal infection was common in people who were 60 or older, those who had underlying chronic disease, or people with both features. There was moderate morbidity in terms of time in bed, time to return to normal activities, and days off work. 25% of patients returned for a second consultation with the general practitioner, in most because of unsatisfactory clinical progress. Community-acquired LRTI are very common, and the range of causative pathogens is similar to that for community-acquired pneumonia. Existing management strategies seem inadequate.