A prospective eight-month study was carried out in 50 children admitted to hospital for radiologically confirmed community-acquired pneumonia. A potential causative agent of infection was identified in 44 (88%) cases. Using virus isolation, virus antigen detection and enzyme immunoassay serology, respiratory virus infection was diagnosed in 30 (60%) patients. Antibody assays for seven bacteria and antigen detection from serum and urine for Streptococcus pneumoniae produced evidence of bacterial infection in 31 (62%) cases. Streptococcus pneumoniae (38%), respiratory syncytial virus (30%) and Mycoplasma pneumoniae (20%) were the most common causative agents. A mixed infection was diagnosed in 25 (50%) episodes. Nine patients failed to respond to antibiotics within 24 h after onset of treatment. Three of them had a pure viral infection, three a mixed viral-bacterial infection, two a Mycoplasma pneumoniae infection mixed with other bacteria and one a pure Mycoplasma pneumoniae infection. All three Mycoplasma pneumoniae infections were initially treated with penicillin.