The purposes of this study were to compare sweat tests used in diagnosing cystic fibrosis (CF), as performed with the Macroduct collection system, with those utilizing the more laborious quantitative pilocarpine iontophoresis test (QPIT), and to ascertain the efficacy of the Sweat-Chek conductivity analyzer in eliminating some possibly unnecessary chloride analyses. A Macroduct sweat test was performed on one arm and a QPIT on the other on 1090 patients, 93 of whom had CF. Of these, 514 patients (43 with CF) also had a conductivity determination on the Macroduct sweat sample. All subjects were referred to the laboratory of one of us (K.B.H.) for sweat testing. Of the QPIT samples, 0.7% were inadequate, as were 6.1% of those from the Macroduct system. When sodium and chloride concentrations from the two tests were compared, the standard errors of the estimate were 3.90 and 3.85, respectively. Agreement within 8 mEq/L could then be expected with 95% confidence limits. With use of the Sweat-Chek analyzer, no patient with CF was found to have a conductivity of less than 90 mmol/L, whereas 430 (91%) of the non-CF subjects had a conductivity of less than 50 mmol/L. None of those 430 subjects had a sweat chloride value > 32 mmol/L. We conclude that the Macroduct collection system provides results equally as satisfactory as those provided by the QPIT and that the Sweat-Chek analyzer frequently eliminates the necessity of measuring chloride concentrations.