OBJECTIVES: This post-hoc analysis examined the relationship between the EQ-5D index and the visual analog scale (VAS), and pulmonary function measured using percent predicted forced expiratory volume in 1 second (FEV), in patients with cystic fibrosis (CF) participating in the STRIVE clinical trial.   METHODS: In a 48-week randomized, placebo-controlled study of ivacaftor in patients aged ≥12 years with CF and a G551D-CFTR mutation, the EQ-5D (index and VAS) and FEV were measured directly from patients at baseline, 15 days, 8 weeks, and every 8 weeks thereafter.  The EQ-5D index was calculated using the U.S. preference-based algorithm.  Pooling assessments and treatments over time, measures were compared among patients with no (FEV≥90%), mild (70-<90%), moderate (40-<70%), and severe (<40%) lung dysfunction.   Ceiling effects (EQ-5D index=1 or VAS=100) were also assessed. RESULTS: : 157 no lung dysfunction, 419 mild, 572 moderate, 66 severe).  Mean index/VAS scores decreased with worsening lung function (no lung dysfunction: 0.967/90.0; mild: 0.949/84.5; moderate: 0.918/75.1; severe: 0.881/66.4).  The EQ-5D index and VAS were at the ceiling in 67.5% (no lung dysfunction: 80.9%; mild: 73.3%; moderate: 62.1%; severe: 45.5%) and 5.6% (no lung dysfunction: 15.9%; mild: 6.9%; moderate: 2.4%; severe: 0.0%) of observations, respectively.  CONCLUSIONS: In a clinical study of patients 12 years of age and older with CF and a G551D-CFTR mutation, the EQ-5D index and VAS demonstrated monotonic decreases with decreasing FEV