Pulmonary function tests including spirometry, N2 washout and volume of trapped gas (VTG) were obtained in 12 children with cystic fibrosis (CF), 6-18 years of age, before and after inhalation of 0.2 mg salbutamol. 11 children had pathologically increased VTG while the lung clearance index (LCI) was abnormal in 9, V'max 25/TLC (maximal flow at 25% of vital capacity/total lung capacity) in 8, residual volume in 6, but FEV1 in only 2 children. VTG/TLC% increased with age while V'max 25/TLC and FEV1 did not change. Neither was clinical score related to age in our subjects. No improvement in lung function occurred after salbutamol inhalation. VTG proved most sensitive for showing abnormality in children with CF.