Sweat from 2,090 noncystic controls and 203 patients with cystic fibrosis were examined with a modified pilocarpine iontophoresis sweat test (modified from Gibson and Cooke). Only one test per person was included in the results (average of duplicate tests performed on the right and left arm). Mean values and two standard deviations for sodium in sweat from the controls are given for various age groups. The results show a marked age variation. The upper limits obtained for normal sweat sodium obtained emphasize that a common upper limit of 60 mmol/L cannot be used.