Over a 5 year period in Newcastle, 18 new cases of cystic fibrosis (CF) were diagnosed in children who had been screened in the newborn period. In six of these children, the screening programme failed. Four of these children had a normal screen and an additional two had elevated immunoreactive trypsin (IRT), but there were problems with the notification procedure. Three of the children missed by the screening process had a significantly delayed diagnosis; in all three cases the diagnosis of CF was suspected clinically, but a sweat test was delayed because of false reassurance from the fact that the child had been screened for CF. In a fourth case, multiple elevated sweat electrolyte levels were obtained, but the diagnosis of CF was considered to be in doubt because of the normal IRT assay. A sweat test should be performed on any child in whom there is clinical suspicion of CF.