The frequency of mutation delta F508 was determined in all 20 Finnish cystic fibrosis (CF) families with living affected children (19 with pancreatic insufficiency). delta F508 was detected in 18 out of 40 CF chromosomes (45%). At least two different mutations associated with pancreatic insufficiently have occurred in a rare haplotype defined by XV2c, CS.7, KM19 alleles 1 2 2. Geographical clustering of delta F508 and other mutations suggested that a founder effect and genetic drift have influenced the frequency of mutations causing CF in Finland.