Regional lung function (RLF) with Xenon133 was investigated in 40 patients with cystic fibrosis (CF) aged 5-28 years (mean 13) at 1-5 occasions during a 3-year period. The RLF was determined with a 4-collimator system and evaluated with a score based on the following parameters: (1) time of elimination of injected isotope and (2) of inhaled isotope, (3) regional ventilation, (4) regional perfusion and (5) ventilation-perfusion ratios. The results were related to spirometry, X-ray score (according to a modification of Chrispin and Norman) and clinical score (according to Shwachman and excluding X-ray), which were all assessed in the same day. RLF correlated to clinical (P less than 0.01) and radiological score (P less than 0.01) and to residual volume (P less than 0.001) and the ratio between one second forced expiratory volume to vital capacity (FEV%) (P less than 0.01), but much higher correlations were found between X-ray score, clinical score and different spirometric variables. We therefore conclude that RLF can be used in patients too young to cooperate in spirometry but that it is of less clinical value in older patients with CF.