This study will evaluate the efficacy of ivacaftor treatment in subjects with CF 6 years of age and older who have a 3849 + 10KB C→T or D1152H CFTR mutation.

Inclusion Criteria:

• Confirmed diagnosis of CF based on protocol-specified clinical features and at least one of the following: increased sweat chloride level, identification of 2 CF causing mutations, or demonstration of abnormal nasal epithelial ion transport.
• A 3849 + 10KB C→T or D1152H mutation on at least 1 CFTR allele.
• FEV1 ≥40% of predicted and ≤105% of predicted at screening.

Exclusion Criteria:

• A G551D, G1244E, G1349D, G178R, G551S, S1251N, S1255P, S549N, S549R, or R117H mutation.
• History of any illness or any clinical condition that, in the opinion of the investigator, might confound the results of the study or pose an additional risk in administering study drug to the subject.
• Ongoing or prior participation in an investigational drug study within 30 days before the Screening Visit.
• Protocol-specified abnormal laboratory values at the Screening Visit
• For subjects <18 years of age at the Screening Visit, evidence of cataract/lens opacity determined to be clinically significant by the ophthalmologist or optometrist during the ophthalmologic examination (OE) at the Screening Visit.
• Use of any moderate or strong inducers or inhibitors of cytochrome P450 (CYP) 3A, including consumption of certain herbal medications and certain fruit and fruit juices, within 14 days before Day 1.
• Pregnant, breastfeeding, or planning to become pregnant during the study.
• Sexually active subjects of reproductive potential must be willing to use appropriate contraception.

From first dose of study drug up to safety follow-up visit (up to Week 28)