To 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 Delta F508 homozygous twins and siblings. In the majority of patients, cAMP and/or Ca(2+)-regulated Cl(-) conductance was detected in the airways and intestine. Our finding of cAMP-mediated Cl(-) conductance suggests that, in vivo, at least some Delta F508 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 Delta F508 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.