Nasal Potential Difference Measurements are a new diagnostic tool in CF. The hypothesis of this study is that Class IV or V mutations in CFTR will be associated with chloride transport in the NPD. The specific aims are 1) to correlate the magnitude of the change in NPD response t low chloride/isoproterenol/amiloride with CFTR genotype; 2) to expand the understanding of the relationship between potential surrogate markers and genotype in CF; and to substantiate the role of NPD measurement as a valuable research tool. Patients are referred by physicians who find some but not all features of CF, or on the basis of CFTR genotype.