In this study the clinical and hematological characteristics, the transmission pattern and the relative rates of globin chain synthesis were determined in the members of four Sardinian families with 14 patients affected by hemoglobin H disease. The severity of hemoglobin H disease in Sardinian subjects shows a high degree of variability. Clinically it usually appears intermediate between the hemoglobin H disease found in Oriental and Negro populations. Theα;/β; specific activity ratio was 0.42±0.10 indicating an analogous biochemical defect like that described in the Chinese. On the basis of the hematological data andα;/β; ratio, the genetics of hemoglobin H disease in Sardinians seems to follow a pattern similar to that observed in Orientals: one parent showingα;-thalassemia-1 trait and the otherα;-thalas-semia-2 trait. Parent offspring transmission of hemoglobin H disease did occur in 2 out of 6 hemoglobin H matings with spouses carrying theα;-thalassemia-l gene. This observation indicates either a high frequency ofα;-thalassemia trait in Sardinians or a high incidence of inbreeding. In one family the mating of a patient with hemoglobin H disease and a normal person produces 6/6 offspring withα;-thalassemia-1. The genetic implications of this transmission pattern are discussed.