The PCR technique was used in a study of the linkage of cystic fibrosis mutations and a polymorphic (GATT)n repeat in intron 6 of the CFTR gene. Absolute linkage disequilibrium was found between the common delta F-508 mutation and the (GATT)6 allele. This allele was also in linkage disequilibrium with other unidentified mutations in the CFTR gene resulting in the pancreatic insufficient form of disease. The frequency of (GATT)n alleles in the pancreatic sufficient form of CF did not differ significantly from the data obtained in the total population. The significance of the (GATT)n polymorphic repeat for the diagnosis of CF is discussed.