The efficacy and safety of hypertonic saline (HS) in CF patients over 6 years of age has been demonstrated in clinical trials of 2 to 48 weeks' duration. Based on these results, a large randomized, placebo-controlled trial of the efficacy and safety of 7% HS administered twice daily for 48 weeks to infants with CF, 4 to 15 months of age at enrollment, is planned (the Infant Study of Inhaled Saline (ISIS) trial). It is anticipated that 150 infants at up to 16 sites will be enrolled in the ISIS trial.