Nine patients have been observed with homozygous beta thalassaemia in each of whom one parent has a normal level of Hb A2. On the basis of clinical, haematological and globin chain synthesis studies these families have been divided into two groups. Group 1 (six families). Heterozygotes for normal Hb A2-beta thalassaemia in this group showed minimal red cell abnormalities, normal osmotic fragility but imbalanced globin chain synthesis (alpha/beta=1.6), and appear to correspond to previous descriptions of 'silent' beta thalassaemia. Double heterozygotes with high Hb A2-beta thalassaemia have a clinical picture of mild beta thalassaemia intermedia characterized by relatively low levels of Hb F (less than 20%) and gamma chain synthesis. Group 2 (three families). beta Thalassaemia heterozygotes with normal HbA2 levels in this group showed more marked red cell abnormalities, decreased osmotic fragility and more imbalanced globin chain synthesis (alpha/beta=2.5) than those in group I. Double heterozygotes with high Hb A2-beta thalassaemia are more severely affected and are transfusion dependent. Haemoglobin F and gamma chain synthesis are high in these cases. The frequency of normal Hb A2-beta thalassaemia in Greece may be as high as 10% of all beta thalassaemia genes and this poses a significant problem for genetic counselling. Various molecular mechanisms are discussed which could account for the heterogeneity within these disorders.