The frequencies of the delta F508 mutation and haplotypes at the loci linked to the cystic fibrosis (CF) gene have been compared in adult CF patients with very mild and with severe lung disease. In patients who are compound heterozygotes for the delta F508 mutation, or who lack the mutation on both chromosomes, the as yet undefined mutations may influence the severity of lung involvement. In patients homozygous for the delta F508 mutation, non-genetic factors cannot fully account for variation in the severity of lung disease. Genes outside the CF locus may influence clinical expression of the disease.