Pneumonia causes 3.2 million deaths per year in children under 5 years old according to the World Health Organization. In spite of the number of deaths, no single clinical or radiological definition for the diagnosis of pneumonia is widely accepted. To determine the extent of agreement between clinical and radiographic diagnoses of pneumonia, we compared the clinical diagnoses made by an experienced paediatrician with diagnoses based on a paediatric radiologist's interpretation of chest radiographs. In 226 children with respiratory illness brought to a hospital outpatient department in Lesotho, pneumonia was the clinical diagnosis for 39 and the radiographic diagnosis for 40; however, for only 19 children did the 2 diagnoses concur. Children with a radiographic diagnosis of pneumonia tended to have been ill longer, to be older, and to be more likely to have a technically adequate radiograph than children with negative radiographs, independent of the clinical diagnosis. In this comparison, radiographic and clinical diagnoses for pneumonia differed substantially. Some of this discrepancy may be explained by misinterpretation of suboptimal films and different rates of evolution of radiographic and clinical manifestations of pneumonia. Radiographic studies and clinical evaluations produced complementary data in this evaluation: of the 60 children with clinical or radiographic evidence of pneumonia, 14 would not have been treated with an antimicrobial drug without radiography. Wider availability of radiography is needed to supplement clinical examination for the diagnosis of pneumonia, and may be particularly valuable in children more than 18 months old, those who have been ill for more than 6 d, and those with fever.