During 12 months in 1981-82, 201 children were hospitalized due to radiologically verified definite or probable pneumonia. In 1985, 194 chest radiographs (anteroposterior views) were re-evaluated jointly by two radiologists, and classified into three categories: alveolar, interstitial and probable pneumonia. In 127 cases definite pneumonia was diagnosed on both occasions, alveolar in 48 cases and interstitial in 79 cases. Variation between the two evaluations 3 years apart was observed in 46 (24%) of the 194 cases; the adjusted kappa (0.47) was in the modest region. Factors contributing to this variation were young age, less than 12 months, and the presence of interstitial infiltration, bronchial obstruction and low C-reactive protein. Factors associated with less marked variation were the presence of alveolar infiltration, auscultatory fine rates and elevated C-reactive protein. The microbial aetiology of infection, assessed by viral and bacterial antigen and antibody assays, showed no association with diagnostic variation. A lateral view of the chest radiograph was obtained from 158 patients; it was positive in 99 (91%) of the 109 cases with definite pneumonia. In only three cases the diagnosis was based on the lateral view alone. Our results show that the radiological diagnosis of pneumonia is difficult in children, especially in young children with interstitial pneumonia.