Thailand deletion of ␣-Thalassemia (thal) 1 involves the 2-, 1-, ␣2-, ␣1-, and 1-globin genes. In Southeast Asians and Taiwanese, this mutation is the second most common long-segment deletion of two ␣-globin genes, after the Southeast Asian deletion. To define the Thailand deletion breakpoints, we used polymerase chain reaction (PCR) to amplify the normal-sequence DNA fragments across the breakpoints. The amplified products were sequenced directly or after cloning into pGem®-3Z or pCR®2.1 vectors. Comparison of the normal and mutant sequences revealed that the 5 breakpoint lies between nucleotides 1,269 and 1,290 upstream of the initiator codon adenine of the 2-globin gene, and the 3 breakpoint lies between nucleotides 29,387 and 29,408 downstream of it. A total of 30,677 nucleotides were deleted. Both breakpoints mentioned above lie within the Alu repetitive sequences and an extensive sequence homology is present around the two breakpoints. These findings suggest that homologous recombination is the mechanism by which the deletion occurs. Based on our data, we used three oligonucleotide primers to amplify the regions across the deletion and its corresponding normal sequence. The feasibility of PCR diagnosis was confirmed in 20 carriers with this deletion.