OBJECTIVE. The aim of this study was to estimate the significance of nasal potential difference (NPD) in the diagnosis of cystic fibrosis (CF) in children with clinical symptoms suggestive of the disease, positive sweat test results, and/or genetically confirmed diagnosis. MATERIAL AND METHODS. NPD measurements according to the modifications by Alton were performed in 50 children with clinical CF symptoms supported by positive sweat test results, 50 children with other obstructive lung diseases, and 50 healthy children. A subgroup of 17 children with the diagnosis confirmed by 2 identified mutations in the CF transmembrane regulatory gene was analyzed individually. RESULTS. The mean NPD value recorded in 50 children with clinical symptoms of CF supported by positive sweat test results and/or genetic analysis was -28.0 mV [SD, 10.2]. The mean NPD value in the subgroup of children with 2 identified mutations in the CF gene (n=17) was more negative than in the subgroup of children with unrecognized mutations (n=33) (-37.1 mV [SD, 7.0] vs. -23.4 mV [SD, 8.3], P<0.001). The mean NPD value in patients with other obstructive lung diseases and healthy children was significantly more positive than in the group of CF children with positive sweat test results and/or identified mutations (-18.1 mV [SD, 3.6] and -15.5 mV [SD, 4.3] vs. -28.0 mV [SD, 10.2], P<0.001). The NPD cut point value for the genetically confirmed diagnosis of CF was -35.0 mV (sensitivity, 93.9%; specificity, 88.2%), while in general, the NPD prognostic value was -24.0 mV (sensitivity, 58.0%; specificity, 98.0%). CONCLUSIONS. The NPD measurement is a valuable tool for the diagnosis of CF in children, but further studies are necessary to establish NPD values related to the CF genotype and to reduce the intrasubject variability of this test.