We have performed an extensive mutation analysis on 184 CF families in Wales. In our previous study, mutations on 329/369 CF chromosomes were identified after screening for delta F508 and sixteen other mutations. To identify the mutations on the remaining 40 uncharacterized CF chromosomes, we have carried out direct DNA sequencing over the complete coding region, intron splice sites, and part of the promoter region of the CFTR gene. During this study we have designed a set of internal sequencing primers which allow clear sequencing through the aforementioned regions. Sequence analysis revealed 15 further mutations (4 of which are novel), and 10 previously described polymorphisms. In total, we have identified 29 mutations, the distribution of which provides further insight into the functional domains of the CFTR protein. We have characterised 99.5% of the CF chromosomes (365/367, one sample degraded). In order to ascertain accurate frequency data for the Welsh population, CF families with at least 3 'Welsh' grandparents were strictly regarded as 'Welsh'. Of these 91 families, delta F508 accounts for 71.6%, 621 + 1G-->T 6.6% and 1898 + 1G-->A 5.5%. The implications for CF population screening in Wales are discussed.