Recent standardisation of the rapid thoraco-abdominal compression technique (RTC) for measuring the maximal expiratory flow at functional residual capacity (V'), and collation of data from 455 healthy infants (male=224) measured on 654 occasions in 3 centres internationally, has led to the availability of gender-specific reference data with which to compare the lung function of infants with suspected airway disease. The airway function of infants with cystic fibrosis (CF) was compared to these reference data in order to assess lung function at diagnosis. V' was measured in newly diagnosed CF infants recruited from 5 specialist centres as part of an ongoing prospective study of longitudinal changes in airway function (London Collaborative Cystic Fibrosis Study). Infants were eligible if they were less than 2 years of age and if there were no additional congenital abnormalities. V' was measured according to ERS/ATS standards and the values compared to the reference population and expressed as SD-scores (Z-score). The CF infants were subdivided into those with (R) and without (NR) a history of respiratory tract infections. 32 infants were recruited (male=14). The median (range) postnatal age at diagnosis (corrected for gestational age) was 9 (-4 to 88)wks. The median (range) age, length and weight at test were 29 (6 to 93)wks, 66.3 (54.0 - 86.8)cm and 6.4 (3.7 - 11.4)kg respectively. Infants were studied within 3 months of diagnosis. Data were obtained in 30 infants. The mean (95% CI) Z-score for V' in these CF infants was -0.90 (-1.37, -0.43). The mean (95% CI) Z-score for the R group (n=17) and the NR group (n=13) were -1.13 (-1.83, -0.42) and -0.61 (-1.24, 0.00) respectively, both of which were significantly lower than the reference population. Many CF infants have impaired airway function at diagnosis. Even those CF infants presenting without a history of respiratory tract infection may have suboptimal airway function soon after diagnosis.