A Bam HI restriction fragment length polymorphism in the interzeta hypervariable region (IZ HVR) of the zeta-alpha gene cluster was used for the prenatal diagnosis of a pregnancy at risk for Hb H hydrops fetalis. The parents had zeta-alpha thalassemia 1 and non-deletion alpha thalassemia, respectively, and a previous hydrops was missed using the conventional method of gene detection. In this prenatal diagnosis, linkage to IZ HVR was used to exclude non-deletion alpha thalassemia, and the numbers of zeta and alpha genes in the fetus were quantitated to predict the exact genotype. Confirmation was made by analysis of cord blood at delivery.