We studied five unrelated Mexican carriers of the Spanish (δβ)(0)-thalassemia [(δβ)(0)-thal] mutation to characterize the size of the deletion, the 5' and 3' breakpoints and the 5' β-globin haplotype. Sequence analysis revealed the presence of an 89,548 bp deletion. The δ- and β-globin genes, two olfactory receptor genes (OR51V1 and OR52A1) and two pseudogenes (OR52Z1P and OR51A1P) were deleted. The 5' breakpoint was located at the same position as previously reported, and the 3' breakpoint was situated 7.0 kb downstream of OR52A1 and 11.7 kb upstream of OR52A5. The Spanish (δβ)(0)-thal allele was associated with the 5' haplotype 2 [- + + - +] in the studied patients. Because this mutation is relatively frequent in Spain, and the Mexican population contains a high level of Spanish genetic background, we propose that the mutation in both populations share a common ancestral origin.