In order to document the frequency and causes of elevated sweat chlorides we reviewed all sweat chloride determinations performed over a 2-year period. Seven hundred and thirty-five quantitative tests were performed. Three hundred and three positive or borderline results were obtained from 133 patients. Thirty-three of these patients did not have clinical evidence of CF. Fourteen of them (42%) suffered from malnutrition or growth stunting when tested. The remaining patients who were normally nourished, represented a very heterogeneous group with no unifying diagnosis. Initial sweat chlorides in 11/14 malnourished children were in the abnormal range (greater than 60 mmol/l). These tended to revert to normal coincident with improvement in nutritional status. Less remarkable elevations in sweat chloride levels were noted in the normally nourished patients, since almost 50% (9/19) were in the intermediate range (50-60 mmol/l). We conclude that malnutrition can be the cause of elevated sweat chlorides, which may lead to an erroneous diagnosis of cystic fibrosis. Clinicians should be aware of this association and adhere to strict criteria for the diagnosis of cystic fibrosis. Also, repeated sweat chloride determinations should be obtained, preferably following re-nutrition.