AbstractIn this study we have defined the spectrum of the β‐thalassemia mutations, the β‐thalassemia haplotypes, and the genotype‐to‐phenotype correlations in a large number of patients with different β‐thalassemia conditions. Seventeen different (β‐thalassemia mutations were detected which included one chromosome each with Hb Dhonburi and Hb Lepore. Five alleles, namely, codon 39 (C→T), IVS‐l‐110 (G→A), IVS‐l‐6 (T C), IVS‐ll‐745 (C→G), and IVS‐l‐1 (G→A), account for 90% of all β‐thalassemia mutations in 846 thalassemic chromosomes studied.Haplotyping for a large number of subjects showed that the five common mutations are linked to a few haplotypes. The presence of milder mutations, mainly IVS‐l‐6 (T C), in about 19% of our patients explains some of the clinical variables. Among the 37 patients with thalassemia of intermediate severity, only 6 were homozygous or compound heterozygous for two severe alleles. The type of β‐thalassemia is the main factor responsible for differences in the phenotypic expression of the disease in patients with Hb S‐β‐thalassemia; patients with Hb S‐β+‐thalassemia are less severely affected than those with Hb S‐β°‐thalassemia. The five most frequent mutations hav