The Boehringer Mannheim test (BM test) for cystic fibrosis has been applied to 15 734 neonates. A positive result occurred in 130, seven of whom were subsequently shown to have the disease. To our knowledge no cases have been missed. Tests have been performed centrally by experienced laboratory staff and the false-positive rate of 0.83% is considered acceptable. It is important to follow up weak as well as strongly positive reactions to the BM test strip.