DNA analysis of a Northern Thai family with a child affected with beta-thalassemia major revealed a novel deletion of 3.4 kb removing the entire beta-globin gene in the proposita and her mother. Detailed mapping of the deletion located the 5' breakpoint in the region between nucleotides -810 and -128 of the beta-globin locus, and the 3' breakpoint between the Ava II and Xmn I sites located downstream of the beta-globin gene. The father transmitted a codon 17 nonsense mutation, a beta-thalassemia variant common in Thailand, to the child.