The Sicilian type of (delta beta) (0)-thalassemia characterized by a approximately 13 kb deletion, was present in a Turkish boy who is a homozygote and in his heterozygous parents who are first cousins. The father with approximately 21% Hb F had five alpha-globin genes (alpha alpha/alpha alpha alpha) and the mother with approximately 10% Hb F had an alpha-thal-2 heterozygosity (alpha alpha/-alpha). The difference in Hb F level is explained by a decreased formation of alpha 2 gamma 2 tetramers in the mother with an alpha-chain deficiency while the extra alpha-globin gene in the father will promote Hb F production.