In Thailand alpha-thalassemia (thal), beta-thal, hemoglobin (Hb) E and Hb Constant Spring (Hb CS) are prevalent. The incidences are 20-30% for alpha-thal (3.5% for alpha-thal-1 and 16% for alpha-thal-2), 3-9% for beta-thal, up to 54% for Hb E and nearly 8% for Hb CS. Different combinations of these genes result in a spectrum of thalassemia syndromes ranging in severity from asymptomatic to intrauterine death. From the known gene frequencies the numbers of thalassemic patients per generation (total population of 50 million) are as follows: Hb Bart's hydrops fetalis 20,000; homozygous beta-thal 31,250; beta-thal/Hb E disease 162,500; Hb H disease (two genotypes) 200,000, making a total of 413,750. In addition, individuals may carry more than two of the abnormal genes leading to complex thalassemia syndromes such as alpha beta-thal, AE-Bart's and EF-Bart's diseases.