The BglII polymorphism near the constant region of the T cell receptor beta chain (TCR c beta) has been investigated in normal controls, patients with cystic fibrosis (CF), and CF carriers. A significant increase was found in the frequency of the 10.0:9.2 kb heterozygous genotype in the CF carrier group (71%) as compared with normal controls (44%) (p = 0.005). Patients with CF also showed an increased frequency of the heterozygous genotype but this was not significant. These results represent a previously unreported disease association and suggest that there may be some form of epistatic interaction between the CF gene and the TCR beta genes such that the double heterozygote is immunologically advantaged.