A collaborative study including 16 cystic fibrosis centers, 22 observers, and more than 1,700 independent observations was undertaken to validate the reproducibility of a scoring system for evaluation of chest radiographs of patients with cystic fibrosis. Five large and 10 small centers were randomly selected by the project statistician. A total of 40 pairs (frontal and lateral) of chest radiographs representing varying degrees of severity was selected by a physician not involved in scoring. Duplicates of these films, identified only by code number, were scored independently by pediatric clinicians (small centers) and by both clinicians and radiologists (large centers). Data collection was complete for all observers. Analysis of the data confirms a moderately high degree of reproducibility by and among observers and indicates that this scoring system could be used in the assessment of patients with cystic fibrosis.