As the effect on mucociliary clearance was found to be better in areas with adequate ventilation, it is logical to assume that treatment with HS may be most efficacious when initiated early in the disease process. So far, no data on the tolerability of inhalation of HS are available for infants with cystic fibrosis. However, evidence from infants with AIDS as well as recent studies in infants with bronchiolitis suggest that hypertonic saline can be safely administered by inhalation in infants. Nevertheless, proof of tolerability in CF infants is a prerequisite for longer term studies of HS in this age group. In older children, tolerability has been tested by measuring pulmonary function both before and after inhalation of HS saline. Similar data are not yet available for infants.