For the diagnosis of cystic fibrosis, we use the "Macroduct Sweat Collection System" (Wescor) to sample sweat, and anion-exchange chromatography to determine its chloride content. The chromatographic system consists of a Whatman Partisil SAX 10-µm column, an isocratic phosphate-buffered mobile phase, and ultraviolet detection at 200 nm. The sweat chloride concentrations of 10 individuals with cystic fibrosis ranged from 101 to 131 mmol/L (mean 111, SD 9 mmol/L). For 11 controls the values ranged from 13 to 54 mmol/L (mean 28, SD 11 mmol/L). We conclude that the procedure is reliable for the diagnosis of cystic fibrosis.