We have found records of 1014 Irish cystic fibrosis patients alive by December 1994, belonging to 883 families. Prevalence in the population is 1/3475 and incidence at birth 1/1461, with a gene frequency of 2.6%. Twenty percent of the patients are aged over 20 years, but at present survival rate falls rapidly after that age. We have identified 85% of the mutations on the CFTR gene in a sample of 29% of the families (506 CF chromosomes). Mutation delta F508 is found in 72% of Irish CF chromosomes, G551D in 6.9%, and R117H in 2%. These are the highest frequencies reported for the latter two mutations world wide. Another seven mutations are found in an additional 4% of CF families. We present new microsatellite haplotype data that could be useful for genetic counselling of CF families bearing some of the 15% of CF mutations still unidentified, and comment on possible uses of our database.