During a 2-year period 159 malnourished children ages 3 months to 5 years with radiologic evidence of pneumonia were investigated to determine the cause of their pneumonia. In addition 119 malnourished children without pneumonia, 119 well-nourished children with pneumonia and 52 well-nourished children without pneumonia were studied as controls. Percutaneous lung aspiration was performed on 35 malnourished and 59 well-nourished children with pneumonia. Bacteria were isolated from the blood, lung or pleural fluid of 28 (18%) malnourished children with pneumonia, 42 (35%) well-nourished children with pneumonia and from the blood of 5 (4%) malnourished children without pneumonia. Streptococcus pneumoniae and Haemophilus influenzae, which were the two organisms isolated most frequently in both groups of children with pneumonia, were found in 17 (11%) malnourished and 39 (33%) well-nourished children with pneumonia. Mycobacterium tuberculosis was detected in 5 malnourished children with pneumonia. A potentially pathogenic virus was identified in 35% of malnourished children with pneumonia and 40% of well-nourished children with pneumonia, and from 25% of children without pneumonia. The viruses identified most frequently were adenovirus and respiratory syncytial virus. There were 26 viral isolates from lung aspirate specimens. Respiratory syncytial virus was more frequently isolated from well-nourished children, while herpes simplex was more often found in malnourished children. Both bacterial and viral agents were identified in 25 children. In situations where measles and kwashiorkor are uncommon, the likely bacterial causes of community-acquired pneumonia in both well-nourished and malnourished children are S. pneumoniae and H. influenzae but, in malnourished children, particularly those with edema, the possibility of other causes, particularly M. tuberculosis, must always be kept in mind.