The incidence of cystic fibrosis (CF) in Finland, 1:25,000 newborn, is one of 
the lowest in Caucasian populations. The delta F508 mutation accounts for 18/40 
(45%) of CF chromosomes in Finland. Other mutations were therefore sought among 
the remaining 55%. Twelve out of 40 chromosomes (30%) were found to carry 
394delTT, whereas G542X and 3732delA were each detected in one chromosome. Eight 
mutations remained unidentified using a testing panel for 26 mutations. Mutation 
394delTT was associated exclusively with haplotype 23-36-13. Five unknown 
mutations were associated with different haplotypes for microsatellite markers, 
whereas three shared the same haplotype. Most delta F508 mutations and all 
unidentified mutations originated from regions of old and dense settlement in 
the coastal regions, whereas 394delTT was geographically clustered and enriched 
in a rural location, consistent with a local founder effect. The remote location 
of Finland and her population history give a plausible explanation for the 
rarity of CF in Finland.