ABSTRACT Cystic fibrosis (CF) is most frequently associated with deletion of phenylalanine at position 508 (∆F508) in the CF transmembrane conductance regulator (CFTR) protein. The ∆F508‐CFTR mutant protein exhibits a folding defect that affects its processing and impairs chloride‐channel function. This study aimed to determine whether CFTR fragments approximately half the size of wild‐type CFTR and complementary to the portion of CFTR bearing the mutation can specifically rescue the processing of endogenous ∆F508‐CFTR in vivo. cDNA encoding CFTR fragments were delivered to human airway epithelial cells and mice harboring endogenous ∆F508‐CFTR. Delivery of small CFTR fragments, which do not act as chloride channels by themselves, rescue ∆F508‐CFTR Therefore, we can speculate that the presence of the CFTR fragment, which does not harbor a mutation, might facilitate intermolecular interactions. The rescue of CFTR was evident by the restoration of chloride transport in human CFBE41o‐ bronchial epithelial cells expressing ∆F508‐CFTR in vitro. More important, nasal administration of an adenovirus expressing a complementary CFTR fragment restored some degree of CFTR activity in the nasal airways of ∆F508 homozy‐gous mice in vivo. These findings identify complementary protein fragments as a viable in vivo approach for correcting disease‐causing misfolding of plasma membrane proteins.—Cormet‐Boyaka, E., Hong, J. S., Ber‐diev, B. K., Fortenberry, J. A., Rennolds, J., Clancy, J. P., Benos, D. J., Boyaka, P. N., Sorscher, E. J. A truncated CFTR protein rescues endogenous ∆F508‐CFTR and corrects chloride transport in mice. FASEB J. 23, 3743‐3751 (2009). www.fasebj.org