A national survey was conducted of patients with cystic fibrosis who were known to paediatricians, chest physicians, and others or whose deaths were reported through the death certification authorities in the United Kingdom during 1977 to 1985. From this population based study a revised incidence figure of one affected baby in 2500 live births was produced. Mortality was very high in the first year of life (7.6%) and was substantially greater for females than for males under age 20 years. A temporal improvement in mortality was found during the period under study, with about 100 more births than deaths occurring each year. This improvement was notable in the first five years of life. Meconium ileus, which used to be a primary cause of early mortality, is becoming increasingly rare as a cause of death. The total prevalence of cystic fibrosis in the UK in mid-1985 was estimated to be about 5000.