Applying the sweat-test as the first choice of test when a diagnosis of cystic fibrosis is suspected is still common practice and advisable. Since the cloning of the CFTR gene more than 400 different cystic fibrosis (CF) mutations have already been identified. The use of CF mutation analysis for diagnostic purposes in CF has therefore remained elusive so far. It is advised to perform sweat-tests as previously described by Gibson and Cooke. In this study we have re-evaluated the results of sweat-tests of 1905 subjects performed in our hospital over a period of 9 years (1983–1992). In 1825 subjects where the CF diagnosis was not made, the mean sodium value obtained was 15.5 ± 9.2 mmol/1. The upper limit of the normal range (2 SD above the mean) is 34 mmol/l. Re-examination of all 239 sweat sodium values (80.9 ± 19.5 mmol/1) in 80 newly diagnosed CF patients (all: Na+ 70 mmol/1) revealed that 5% of the values were below 50 mmol/1, the lowest sweat value obtained being 27 mmol/1. Based on these results, we recommend in case of clinical suspicion of CF and sweat values above 30 mmol/1 to repeat the sweat-test and to determine both sodium and chloride for optimal discrimination.