High-resolution computed tomography (HRCT) is widely used to assess pulmonary parenchymal disease in adults. The authors used ultrafast CT with 3-mm collimation and a 100-msec scan time to obtain HRCT scans in 36 children (mean age, 49.4 months). Clinical diagnoses included normal lungs (n = 6), cystic fibrosis (n = 12), obliterative bronchiolitis (n = 6), idiopathic pulmonary hemosiderosis (n = 2), and other lung diseases (n = 10). The HRCT scans and chest radiographs were reviewed separately in blinded fashion. Pulmonary parenchymal abnormalities were categorized into interstitial, airspace, and airway processes. Nine patients with normal chest radiographs had abnormal HRCT scans. In five other patients, the extent of abnormal lung parenchyma was considerably greater on HRCT scans than on chest radiographs. HRCT scans allowed accurate characterization of the type of lung process in 24 of 30 patients when compared with clinical or biopsy findings. Early HRCT findings in cystic fibrosis included lobular air trapping, bronchial wall thickening, and centrilobular nodules. The use of cine CT with high-resolution techniques is feasible in children too young or too sick to hold their breath. HRCT may enable early detection and characterization of pulmonary disease and depiction of the extent of lung abnormality.