To assess the relation between genotype and severity of disease in cystic fibrosis (CF) the frequencies and extent of several features of its phenotypic expression were investigated in the 235 patients who attend the Danish CF Centre. 14 patients who attend irregularly and 3 who do not carry the delta F508 mutation at all were excluded. The case-reports of the remaining 218 patients (aged 4 months to 41 years) were carefully evaluated, and they were all analysed for the delta F508 mutation. 172 (79%) were homozygous for delta F508 and 46 (20%) were heterozygous. The mutation therefore occurs on 89% of the chromosomes analysed. There were no significant differences between the homozygous and heterozygous groups in the proportions with meconium ileus at birth, liver involvement, or chronic Pseudomonas aeruginosa infection. However, significantly more of the homozygous patients had onset of symptoms before the age of 6 months (p less than 0.025); they were significantly younger at diagnosis (p = 0.013) and centre referral (p = 0.006); they required greater pancreatic enzyme substitution (p = 0.0002) and had poorer lung function; and the calculated yearly incidence of chronic Ps aeruginosa infection and yearly mortality rates were greater than in heterozygous patients (p = 0.0001).