A thorough survey was conducted of patients with cystic fibrosis living in the UK from 1977 onwards. This was supplemented with death certificate data back to 1968. There was one case identified for each 2475 births. The total number of cases in the UK rose linearly from 4086 in 1977 to 5426 in 1988 and is estimated to be 6000 in 1992. The probability of survival improved in the period under study for all age groups and in both sexes. It was worst in the first year of life, dropped sharply thereafter but then increased steadily with age. There has been a very marked improvement over the years in survival in the first year of life, but a lesser one in older age groups. Male mortality was greater than that in females in the first year of life, but less in subsequent years giving, overall, worse survival for females. A set of cohort and 'current' survival graphs for the two sexes are given.