Acute respiratory infections cause four and a half million deaths among children every year, the overwhelming majority occurring in developing countries. Pneumonia unassociated with measles causes 70% of these deaths; post-measles pneumonia, 15%; pertussis, 10%; and bronchiolitis and croup syndromes, 5%. Both bacterial and viral pathogens are responsible for these deaths. The most important bacterial agents are Streptococcus pneumoniae, Haemophilus influenzae, and Staphylococcus aureus. The data on bacterial etiology of pneumonia during the first 3 months of life are limited, and almost no information on the role of chlamydia and pertussis in this age period is available. The distribution of viral pathogens in developing countries can be summarized as follows: respiratory syncytial virus, 15%-20%; parainfluenza viruses, 7%-10%; and influenza A and B viruses and adenovirus, 2%-4%. Mixed viral and bacterial infections occur frequently. Risk factors that increase the incidence and severity of lower respiratory infection in developing countries include large family size, lateness in the birth order, crowding, low birth weight, malnutrition, vitamin A deficiency, lack of breast feeding, pollution, and young age. Effective interventions for prevention and medical case management are urgently needed to save the lives of many children predisposed to severe disease.