Recently a few cystic fibrosis (CF) patients with borderline or normal sweat tests have been reported. These patients present a diagnostic challenge. We aimed to study the sweat Cl/Na ratio in cystic fibrosis patients and to assess whether this ratio could be used as a diagnostic criteria. The mean sweat Cl/Na ratio of 3 groups was compared: Group A: 71 CF patients carrying 2 mutations known to be associated with severe disease presentation (delta F508, W1282X, G542X, N1303K, 1717-1G --> A). Group B: 10 compound heterozygous patients who carry one mutation associated with mild clinical disease (3849 + 10 kb --> T). Group C: 142 normal subjects. Sweat chloride levels higher than those of sodium were found in 96% of patients in Group A as compared to 3% of patients in Group C. In Group B 40% of the patients had sweat chloride levels higher than or equal to sodium levels. The mean Cl/Na ratio of Group A (1.2 +/- 0.1) differed significantly from that of Group B (0.94 +/- 0.1) and both groups had significant higher mean Cl/Na ratio compared to Group C (0.7 +/- 0.4) (P < 0.001). Thus in individuals with a borderline sweat test and a Cl/Na ratio > or = 1 the diagnosis of CF should be considered. However, a Cl/Na ratio < 1 does not exclude CF, since patients carrying mild mutations may have sweat sodium levels higher than those of chloride. Our findings suggest that the sweat Cl/Na ratio in CF is genetically determined and it may be of help in establishing the diagnosis of CF in patients with a borderline sweat test.