Patients attending cystic fibrosis clinics had sweat sodium and chloride concentrations measured, were reassessed clinically and had DNA studies performed. Sweat test results were compared with a matched control population. In both populations sweat sodium increased with age up to 12 years, and did not change significantly thereafter. The age-related increase was significantly less in the cystic fibrosis group. Sweat chloride increased with age in normal, but not in cystic fibrosis children. After age 12 years there was no age-related change in the normal group, and a fall with age in the cystic fibrosis group. Sweat chloride provided the best discrimination between normal and cystic fibrosis populations and this was particularly important in older subjects. Combining sweat sodium and chloride results did not improve discrimination. Nine patients were identified with equivocal sweat chloride results. DNA studies showed six of these subjects were heterozygous for the delta F508 mutation in the cystic fibrosis gene. Clinical assessment did not always resolve cases with borderline sweat chloride results.