To assess whether this association was independent of CFTR genotype, we examined white patients in the largest sample of CF subjects (CFF) who had identical CFTR genotypes (F508del homozygotes) and found that warmer temperatures remained associated with lower lung function with a similar co-efficient (−0.35; n = 6367; p<0.001) to that from the entire CFF sample (−0.31). The gene discussed is CFTR; the disease is cystic fibrosis.