Infant Study of Inhaled Saline in Cystic Fibrosis

Inclusion Criteria: * Diagnosis of CF by newborn screening or at least one clinical feature of CF, AND either: (a) A documented sweat chloride ≥ 60 mEq/L by quantitative pilocarpine iontophoresis or (b) A genotype with two identifiable CF-causing mutations * Informed consent by parent or legal guardian * Age 4 months to \< 60 months at Enrollment visit. If participating in Infant Pulmonary Function testing (selected sites), age 4 months to \< 16 months at Enrollment visit. * Ability to comply with medication use, study visits, and study procedures as judged by the site investigator Exclusion Criteria: * Acute intercurrent respiratory infection, defined as an increase in cough, wheezing, or respiratory rate with onset in 1 week preceding Enrollment visit * Acute wheezing at Enrollment visit (prior to HS test dose), or at Infant PFT visit (prior to infant pulmonary function testing), as applicable * Oxygen saturation \< 95% (\< 90% in centers located above 4000 feet elevation) at Enrollment visit (prior to HS test dose) or at Infant PFT visit (prior to infant pulmonary function testing), as applicable * Other major organ dysfunction, excluding pancreatic dysfunction * Physical findings that would compromise the safety of the subject or the quality of the study data as determined by the site investigator * Investigational drug use within 30 days prior to Enrollment visit, or within 30 days prior to Infant PFT visit as applicable * Treatment with inhaled hypertonic saline at any concentration within 30 days of Enrollment visit, or within 30 days prior to Infant PFT visit as applicable * Chronic lung disease not related to CF * Intolerance of test dose of HS at Enrollment visit * A sibling that has been randomized and is still enrolled in ISIS002 Additional Exclusion Criteria for Participation in Infant Pulmonary Function Testing: * History of adverse reaction to sedation * Clinically significant upper airway obstruction as determined by the Site Investigator (e.g. severe laryngomalacia, markedly enlarged tonsils, significant snoring, diagnosed obstructive sleep apnea) * Severe gastroesophageal reflux, defined as persistent frequent emesis despite anti-reflux therapy * Acute intercurrent respiratory infection, defined as an increase in cough, wheezing, or respiratory rate with onset in 2 weeks preceding visit