In order to determine whether a high proportion of adults with pulmonary diseases have sweat chloride concentrations in the range usually considered diagnostic for cystic fibrosis (greater than 60 mEg/L), we performed the standard diagnostic "sweat test" of Gibson and Cooke prospectively on 187 subjects 18 to 85 yr of age who did not have cystic fibrosis (166 of them had some pulmonary or allergic disorder, and 21 were healthy). In this group, 99% had sweat chloride concentration less than 70 mEq/L, and 96%, less than 60 mEq/L. Those taking steroids had sweat chloride concentration slightly but significantly lower than those who did not take steroids, probably because of the mineralocorticoid effect. Six patients had pancreatitis, and 2 of those had sweat chloride concentration greater than 60 mEq/L, a distribution of values significantly different (p less than 0.005) from the rest of the population. Our results suggest that a very small proportion of adults with pulmonary diseases have sweat chloride concentrations in the range usually considered diagnostic for cystic fibrosis, and that the sweat test is a good discriminant for cystic fibrosis even in the adult age group.