Cystic fibrosis is a common, lethal, genetic disease caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR), and it is an attractive target for gene therapy. Adeno-associated virus (AAV) is a naturally replicationdeficient single-stranded DNA parvovirus. Preclinical studies showed that CFTR transcripts and protein can be detected up to 6 months after transduction with an AAV-CFTR vector, 1 with no pathogenicity, 2 suggesting that AAV performs well as a gene transfer vehicle for CFTR. The maxillary sinuses are attractive for evaluating new treatments of cystic fibrosis because they have ion-transport systems and microbiology similar to those of the lower respiratory tract. 3, 4 Recurrence of maxillary sinusitis may prove to be a surrogate for infectious exacerbations characteristic of cystic fibrosis lung disease. Ten patients who had undergone bilateral endoscopic antrostomies were treated with AAV-CFTR2 in a phase I, randomised, non-blinded, dose escalation protocol. 5 Five patients received one dose of vector and five were treated with an initial low dose followed by a second, higher dose in the contralateral maxillary sinus. The figure reveals semiquantitative PCR of sinus biopsies from patients treated with 100 000 replication units of AAV-CFTR. At this dose, DNA transfer was observed at 0· 1–1 vector copy per cell in biopsies done 14 days after treatment. Persistent DNA transfer was noted in patients 8 and 9, both of whom