Since the identification of the CF gene, less than 3 years ago, progress in analysing the function of its product, the cystic fibrosis transmembrane conductance regulator (CFTR), has been remarkable. It is now clear that CFTR functions as a small conductance chloride channel in epithelial membranes. However, many other questions remain unanswered. How does a defect in this channel result in the various pathologies associated with cystic fibrosis? Does CFTR have additional functions? How do CF mutations alter the function of the protein? Tools are now available to address these and other questions. Many features of CFTR activity suggest that pharmacological interventions may be possible. Nevertheless, an enhanced understanding of CFTR function is still essential before this basic research will provide direct benefit to CF sufferers.