Retrospective analysis of sodium and chloride results from sweat tests carried out in Bristol and Sheffield has shown that misdiagnosis of cystic fibrosis (false negatives and false positives) is considerably less if sodium and chloride are both measured. In patients with cystic fibrosis the chloride concentration is usually higher than the sodium, whereas in normal subjects the reverse usually occurs. This observation is particularly useful when borderline results (50-70 mmol/L) are obtained.