The diagnosis of cystic fibrosis (CF) can be difficult if the sweat test and routine deoxyribonucleic acid (DNA) analysis are inconclusive. Under these circumstances, measurement of nasal potential difference (NPD) was proposed as a complementary diagnostic tool, as demonstrated in subjects bearing the G551S or 3849+10KbC-->T mutations. The purpose of the present study was to verify the diagnostic value of this technique in CF patients with a borderline sweat test. NPD was measured in 18 patients with a borderline sweat test, in whom CF diagnosis was based on the presence of one CF gene mutation in each chromosome (CF borderline). These patients were compared both to non-CF controls and CF patients with an abnormal sweat test (CF controls). Basal NPD values of CF borderline patients (mean value -39+/-6 mV, range -29 to -52 mV; n=18) were in the pathological range of CF controls (-39+/-8 mV, range -28 to -57 mV; n=37), and both were statistically different from values obtained in non-CF controls (-15+/-4 mV, range -6 to -23 mV; n=24; p<0.0001). Mutation analysis confirmed a high frequency of the 3849+10KbC-->T mutation in this group of CF borderline patients (positive in 14 out of 18 subjects), whereas other mutations, such as AF508, Q552X, N1303K and R1162X, were also found to be associated with this atypical CF phenotype. These results confirm the presence of pathological values of basal NPD in CF patients with borderline sweat test, and also extend this finding to subjects bearing genotypes other than the G551S and 3849+10KbC-->T mutations. The present findings, therefore, confirm the usefulness of measurement of basal nasal potential difference in all those patients in whom diagnosis of cystic fibrosis can be suspected but the sweat test remains inconclusive.