Twenty five children aged 8 mth to 17 yr were studied to determine the usefulness of computer assisted ventilation (VP) scanning in analysis of regional pulmonary function in cystic fibrosis. Ventilation and perfusion scintiphotographs were compared with the patients' radiographs. Regional fractional exchanges of air and relative regional VP ratios were obtained. Whole lung air exchange was significantly correlated with standard pulmonary function tests and radiograph scores. Ventilation perfusion ratio gradients were reversed in patients with predominantly upper zone disease, and regional VP ratios were uneven in distribution. Quantitative assessment of regional ventilation and perfusion is a useful technique for assessing severity of pulmonary disease in children with cystic fibrosis.