We carried out alpha-globin gene analysis by restriction endonuclease mapping in a family with 2 cases of HbH disease. These data show that HbH disease in this family results from the interaction between a common deletional defect and a less common non-deletion alpha-thal lesion (--Med/alpha alpha thal). Furthermore, the presence of a beta-thal determinant in this family was investigated by beta gene polymorphism study. We showed that a patient with HbH disease also inherited a beta-thal determinant from the mother and although this was a beta O-thal gene, it was not sufficient to mask the severe alpha chain deficiency. The --Med/alpha alpha thal genotype is more severe than other types of alpha thalassaemia interactions causing HbH disease, probably because the expression of alpha alpha thal determinant may be lower than that of an alpha-thal determinant containing just a single alpha gene (-alpha) and the output so poor that the presence of one beta-thal gene does not significantly change the clinical picture.