A diagnosis of cystic fibrosis was incorrectly made after false-positive sweat tests in 14 children. 13 of these children had been tested at hospitals where it seems likely that sweat tests were not done very often. All the children had normal sweat-electrolyte values when the test was repeated at a regional paediatric centre where approximately 250 sweat tests are done each year. In 5 cases, detailed testing of pancreatic function was normal. None of the children had typical chest disease and only 2 had gastrointestinal symptoms. In the absence of the typical clinical features of the disease a diagnosis of cystic fibrosis should be made with extreme caution and only after meticulous testing of both sweat electrolytes and pancreatic function.