As part of a study of the aetiology of pneumonia in malnourished and well nourished children, 100 children aged between 3 and 58 months with pneumonia were investigated using blood culture and culture of lung or pleural aspirates. In 44 children one species of bacterium was isolated from blood (6), lung culture (30), or both (8), while in eight children two species were isolated. In four of these eight children, one organism was isolated from blood culture, while a different organism was isolated from lung or pleural aspirate, indicating that blood culture does not always reflect the true aetiology of pneumonia. In this group of children with lobar pneumonia or empyema, blood culture alone yielded a bacterial pathogen in 18 (18%) cases, while the addition of percutaneous lung aspiration or pleural aspiration increased the yield to 52 (52%). In experienced hands the procedure of percutaneous lung aspiration is safe. It gave significant therapeutic advantages to the six children in this series from whom Staphylococcus aureus or Mycobacterium tuberculosis were isolated.