Linkage analysis with cloned gene probes has shown that the mutation causing cystic fibrosis is located in the middle of the long arm of chromosome 7. First-trimester diagnosis of cystic fibrosis is reported in four informative families and second-trimester diagnosis in one family with fetal DNA prepared from chorionic villi, hybridised with the tightly linked DNA probes, pJ3.11 and met. Risk calculations show that the expected false-negative and false-positive rates are approximately 2% and 6%, respectively, for typical nuclear families with one affected living child. Existing probes are sufficiently informative to allow full diagnosis in about two-thirds of couples presenting with at least one affected child. In half of the remainder, the inheritance of one parental mutant chromosome can be deduced.