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Abstract: Objectives: The aim of this study was to update the incidence data of beta thalassaemia mutations in various populations and compare it to the spectrum of mutations in the United Kingdom (UK) population in order to determine the impact of immigration. Design and methods: Published data for the β-thalassaemia mutation spectrum and allele frequencies for 60 other countries was updated and collated into regional tables. The β-thalassaemia mutations in the UK population have been characterised in 1712 unrelated carriers referred for antenatal screening. Similarly, the α-thalassaemia mutations in the UK population have been characterised in 2500 possible α-thalassaemia carriers. Results: A total of 68 different β-thalassaemia mutations were identified in couples requiring screening for antenatal diagnosis in the UK population. Of these mutations, 59 were found in immigrants to the UK, from all major ethnic groups with a high incidence of haemoglobinopathies. A total of 40 different α-thalassaemia mutations were characterised in the UK population. Ten deletion mutations were identified, including all the Southeast Asian and Mediterranean α0-thalassaemia mutations. In addition, 30 non-deletion α+-thalassaemia mutations were discovered, accounting for 46% of the worldwide known non-deletion mutations. Conclusions: The impact of immigration has resulted in the UK population having a higher number of β-thalassaemia mutations and α-thalassaemia mutations than any of the 60 other countries with a published spectrum of mutations, including both endemic countries and the non-endemic countries of Northern Europe. The racial heterogeneity of the immigrant population in a non-endemic country significantly increases the spectrum of haemoglobinopathy mutations and their combinations found in individuals, making the provision of a molecular diagnostic prenatal diagnosis service more challenging. [Copyright &y& Elsevier]