Summary. Our paper describes two Sardinian families with α‐β thalassaemia interaction. In the first (family S), the propositus, whose haemoglobin pattern at birth consisted of about 25% Hb Bart's and 75% Hb F, successively developed a clinical and haematological picture typical of Cooley's anaemia. Haematological and globin chain synthesis studies together with these findings suggest that he is homozygous for β0 thalassaemia and heterozygous for α thalassaemia‐1 and α thalassaemia‐2. This conclusion is further substantiated by the finding of various combination of α and β thalassemia among his family members. In the P family two twins whose haemoglobin pattern and synthesis data at birth were similar to those of the proband of family S died in the neonatal period. The mother was assumed to be a compound heterozygte for α thalassaemia‐2 and β0 thalassaemia and the father for α thalassaemia‐1 and β0 thalassemia. The homozygous state for β0 thalassaemia in association with the α thalassaemia 1 and α thalassaemia 2 genes results in a severe clinical picture similar to that of a homozygous β0 thalassaemia. The interaction between the heterozygous state for β0 thalassaemia and the α thalassaemia 1 or α thalassaemia 2 genes, or the combination of both, results in a haematological picture similar to that of a β thalassaemia heterozygote. Copyright © 1979, Wiley Blackwell. All rights reserved