Criteria for a positive sweat test in children (sweat sodium greater than 60 mmol/L and sweat chloride greater than 70 mmol/L) were investigated in a series of adults aged between 18 and 40 years using the standard Gibson and Cooke technique of pilocarpine iontophoresis. A significant number of non-cystic fibrosis adults had sweat sodium values greater than 60 mmol/L whereas a concentration of chloride greater than 70 mmol/L always discriminated adults with cystic fibrosis from normal adults and those with chest disease not due to cystic fibrosis. In all cases of adult cystic fibrosis the sweat sodium concentration was greater than 80 mmol/L. The Na:Cl ratio and sum of Na and Cl provided additional helpful criteria in distinguishing between adults with and without cystic fibrosis.