The occurrence of Hb H hydrops fetalis is reported for the first time. The mother has zeta-alpha thalassemia 1 (zeta zeta alpha alpha/----) and the father has non-deletion alpha thalassemia [zeta zeta alpha alpha/zeta zeta (alpha alpha)T]. The complete deletion of the zeta alpha cluster on one chromosome was confirmed by quantitation of alpha and zeta gene numbers, the normal alpha and zeta gene patterns arising from the remaining normal chromosome, and the decreased alpha/beta globin chain ratio of 0.57. The non-deletion alpha thalassemia defect could only be identified by the imbalanced alpha/beta globin chain ratio of 0.65 in the presence of normal gene numbers and patterns. The newborn was markedly anemic, unlike those with classical Hb H disease, because the non-deletion alpha thalassemia defect is more severe than alpha thalassemia 2. The decreased zeta genes during fetal life might have additional deleterious effects. In this family, the distinct BamHI restriction fragment length polymorphism in the hypervariable region of the zeta genes may be used for future prenatal diagnosis.