Hb Q-Thailand [alpha 74(EF3)Asp-->His] is often found in Thailand, China, and other Southeast Asian countries. The alpha-Q-Thailand gene is strongly linked to an alpha gene deletion and has important implications in the identification and diagnosis of hemoglobinopathies and thalassemias. The alpha-Q-Thailand mutation was previously mapped to the alpha 1 gene in a study of Chinese patients. In this paper, a Thai patient with Hb Q-Thailand/Hb H disease and his mother were studied at the DNA level, and the gene organization of Hb Q-Thailand in the Thai patient was found to be the same as that of Chinese patients (i.e. the Hb Q-Thailand gene is located on the alpha 1 gene of chromosome #16, while the -4.2 kb or leftward deletion involves the alpha 2 gene). Also, the GAC-->CAC mutation proposed at codon 74, has been confirmed by DNA sequencing and a simple and accurate method for diagnosis of the Hb Q-Thailand variant has been developed based on restriction enzyme analysis. Since the GAC-->CAC mutation generates new cutting sites for both restriction enzymes Apa LI and Hgi AI, polymerase chain reaction amplification of a specific region around codon 74, followed by digestion with these enzymes and agarose gel electrophoresis of the digested products, permits rapid and accurate identification of Hb Q-Thailand.