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Hemoglobin Constant Spring is an elongation mutation of the alpha 2-globin locus that results in a thalassemic phenotype. It has a high prevalence in Asian populations. When inherited with other alpha-thalassemia determinants, the Constant Spring gene has the potential to cause severe forms of alpha-thalassemia. Accurate diagnosis of the condition with standard hemoglobin electrophoresis is unreliable due to the small to undetectable amounts of the mutant hemoglobin present. Because of the extensive sequence homology of the alpha 1 and alpha 2 loci, allele-specific hybridization to total genomic DNA containing the Constant Spring gene would not distinguish between heterozygous and homozygous hemoglobin Constant Spring. Selective enzymatic amplification of alpha 2-globin DNA sequences, however, allows unambiguous diagnoses to be made using allele-specific hybridization. This method is useful for providing accurate genetic counseling and prenatal diagnosis in populations and specific families in which precise diagnosis is important.