This was a Phase 2, randomized, double-blind, placebo-controlled, multiple-dose study of orally-administered VX-809 in participants with CF who are homozygous for the specific CFTR mutation known as ∆F508 or F508del. Enrollment was planned for 90 participants at approximately 20 centers. Participants were planned to be randomized in a 4:1 ratio to receive 1 of 4 doses of VX-809 or placebo once a day for 28 days in a parallel design. Participants were outpatients during the study, except for overnight stays on Day 1 and 28.