Details of haplotype and delta F508 status from various populations represented in the cystic fibrosis (CF) DNA bank of the Royal Manchester Children's Hospital are provided, together with information on the association of genotype and clinical status. Clinical details and DNA analyses from native English in the North-West and South-West of England (Bath), from Lancashire Pakistani families and from Afrikaans Namibian families are compared. A 78.5% incidence of delta F508 has been found in English families. Compound heterozygotes with CF and only one delta F508 gene have an increased likelihood of having milder disease, with less Pseudomonas isolated from sputum and relatively more showing either no regular respiratory pathogens or colonisation with Staphylococcus. There is also a relative increase in meconium ileus in these compound heterozygotes. The diagnosis of CF may be in doubt in some subjects negative for delta F508. Some of the Bath families have unusual haplotypes for an English population and a compound heterozygote delta F508/delta I507 has been found. There is evidence from metD analysis of the founder effect in the Afrikaans Namibian families, who have a high delta F508 incidence.