Previous work confirmed the abnormal potential difference between the undersurface of the inferior nasal turbinate and a reference electrode in cystic fibrosis, but the technique is difficult and the results show overlap between the cystic fibrosis and the control populations. In the present study the potential difference from the floor of the nose has therefore been assessed in normal subjects, as well as in adult patients with cystic fibrosis, bronchiectasis and Young's syndrome. Voltages existing along the floor of the nasal cavity were recorded. The mean potential difference was similar in controls (-18 (SD 5) mv) and in patients with bronchiectasis (-17 (6) mv) and Young's syndrome (-20 (6) mv). The potential difference in cystic fibrosis (-45 (8) mv) was significantly different from controls (p less than 0.002) and there was no overlap between the cystic fibrosis values and values obtained in normal and diseased controls. This simple technique therefore discriminates well between patients with cystic fibrosis and other populations, raising the possibility of its use to assist in diagnosis.