T o investigate the impact of chloride (Cl^–) permeability, mediated by residual activity of the cystic fibrosis transmembrane conductance regulator (CFTR) or by other Cl^– channels, on the manifestations of cystic fibrosis (CF), we determined Cl^– transport properties of the respiratory and intestinal tracts in ΔF508 homozygous twins and siblings. In the majority of patients, cAMP and/or Ca²⁺-regulated Cl^– conductance was detected in the airways and intestine. Our finding of cAMP-mediated Cl^– conductance suggests that, in vivo, at least some ΔF508 CFTR can reach the plasma membrane and affect Cl^– permeability. In respiratory tissue, the expression of basal CFTR-mediated Cl^– conductance, demonstrated by 30% of ΔF508 homozygotes, was identified as a positive predictor of milder CF disease. In intestinal tissue, 4,4′-diisothiocyanatostilbene-2,2′-disulfonic acid–insensitive (DIDS-insensitive) Cl^– secretion, which is indicative of functional CFTR channels, correlated with a milder phenotype, whereas DIDS-sensitive Cl^– secretion was observed mainly in more severely affected patients. The more concordant Cl^– secretory patterns within monozygous twins compared with dizygous pairs imply that genes other than CFTR significantly influence the manifestation of the basic defect.