Chest X-ray findings were related to virus diagnosis, age and secretory bacterial findings in 128 infants and children under 7 years of age with clinical pneumonia and bronchiolitis. They belonged to a cohort examined in connection with the introduction of rapid methods for virological diagnosis. Seventy-six children had a virus infection diagnosed by examination of nasopharyngeal secretion and/or by serological methods. Thirty-seven of these children were classified as having pathogenic bacteria of importance in the respiratory tract. Four groups were compared: virus infected children with or without bacteria in the secretion and the corresponding virus negative groups. The X-rays were normal significantly more often in the virus positive/bacteria negative group compared with the other groups. Alveolar pneumonia appearing as lobar or segmental consolidations ("lobar" pneumonia) was observed with equal frequency and without relation to bacterial findings in the virus positive and the virus negative groups. But it was more often observed in the respiratory syncytial virus (RSV) infected children under 6 months of age compared with the older RSV children. In comparison disperse alveolar infiltrations ("bronchopneumonia") mostly appeared in the virus positive group. Interstitial pneumonia and peribronchitis were often present together in children over 6 months of age. There was no significant difference in the X-ray appearances in the groups with and without bacterial findings in the tracheal secretion except for a higher frequency of normal chest radiographs in the bacteria negative group.