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Aim: Thalassaemia is a good candidate disease for control by preventive genetic programmes in developing countries. Accurate population frequency data are needed for planning the control of thalassaemia in the high risk Guangdong Province of southern China. Methods: In total, 13 397 consecutive samples from five geographical areas of Guangdong Province were analysed for both hoematological and molecular parameters. Results: There was a high prevalence of carriers of α thalassaemia (8.53%), β thalassaemia (2.54%), and both α and β thalassaemia (0.26%). Overall, 11 .07% of the population in this area were heterozygous carriers of α and β thalassaemia. The mutation spectrum of α and β thalassaemia and its constitution were fully described in this area. This study reports the true prevalence of silent α thalassaemia in the southern China population for the first time. In addition, two novel mutations that give rise to α thalassaemia, one deletion resulting in β thalassaemia, and a rare deletion (- - THAI allele) previously unreported in mainland China were detected. The frequency of the most common mutation, the Southeast Asian type of deletion (- - SEA, accounting for 48.54% of all α thalassaemias) was similar to the total of two α+ tholassaemia deletions (-α3.7 and -α4.2, accounting for 47.49% of α thalassaemia). Conclusion: Both α and β thalassaemia are widely distributed in Guangdong Province of China. The knowledge gained in this study will enable the prolected number of pregnancies at risk to be estimated and a screening strategy for control of thalassaemia to be designed in this area. [ABSTRACT FROM AUTHOR]