A new method of collecting and analysing sweat (Wescor Macroduct) has advantages of simplicity of collection and direct reading of results by osmometry. Forty-seven children with cystic fibrosis and 47 normal children had sweat tests performed simultaneously by the Gibson and Cooke method and by the Wescor Macroduct method. The new method had a higher rate of inadequate collection (19% vs 6%) which was more marked in children under 5 years of age. This was due partly to the difficulty of fitting a 2.5 cm pilocarpine gel disc to small arms. When an adequate collection was obtained, results were reliable with no false negatives occurring during this study. The Wescor Macroduct sweat test is a reliable method for use in peripheral centres to screen patients suspected of having cystic fibrosis. All children with an inadequate collection or a positive result should be referred to a reference centre for confirmation of the diagnosis. However, the majority will be saved the expense and disruption of travel.