We identified and characterized a novel beta(0)-thalassemia mutation due to partial deletion of the 5' end beta-globin gene including the mRNA cap site and a part of exon 1. The deletion was precisely 105 basepair (bp) in length extending from position -24 or -25 to +80 or +81 relative to the beta-globin gene mRNA cap site. This mutation was detected in three individuals from a family originating in the area of southern Thailand. The propositus was a 39-year-old female and noted to be heterozygous for beta-thalassemia with hemoglobin (Hb) level of 10.1 g/dl, MCV 70 fl, MCH 23.1 pg, HbA2 6.3%, and HbF 2.4%. Her son was 9 years of age and was also heterozygous for the mutation, having Hb level of 10.8 g/dl, MCV 58 fl, MCH 19.0 pg, HbA2 5.6%, and HbF 4.3%. Her 6-year-old daughter was affected, having a genotype of this mutation and a G-C transition at IVS 1 nt 5. Although the deletion does not include the beta-globin gene promoter sequences, the individuals heterozygous for this mutation have an elevated HbA2 level slightly higher than observed in most carriers of beta-thalassemia caused by point mutations.