Two homozygousδ0β0-thalassemia patients, one with theGγAγtype and the other with theGγtype, and their heterozygous parents are described. Red cell indices among the heterozygotes with theGγAγtype ofδ0β0-thalassemia were markedly different from those in heterozygotes with theGγtype. However, the imbalance inin vitrohemoglobin synthesis was quite similar in the two heterozygous conditions. The same was observed for the homozygous patients; thein vitrochain synthesis was severely imbalanced as seen inβ-thalassemia major. The clinical and some of the hematological findings were milder in theGγ-δ0β0-thalassemia homozygote than in theGγAγ-δ0β0-thalassemia homozygote. The death of a sibling of theGγ-δ0β0-thalassemia homozygote with a diagnosis of thalassemia major suggests that both types ofδ0β0-thalassemia could follow a severe clinical and hematological course. The discovery of theGγtype ofδ0β0-thalassemia in a Turkish child shows that two types ofδ0β0- thalassemia can be found in that country. Differentiation between the two types can only be made through structural analyses of Hb F.