Hb Constant Spring (Hb CS), the gene (alpha(CS)) of which arises from a point mutation in the termination codon of the alpha2-globin gene, is the most prevalent variety of nondeletional alpha-thalassemia (alpha-thal) in Asian populations. It is a major cause of Hb H disease in compound heterozygotes who have Hb CS combined with a duplicated alpha gene deletion (--/alpha(CS)alpha), and it tends to be more severe than Hb H disease which is caused by a triple alpha gene deletion (--/-alpha). Hb CS is often missed by routine electrophoresis but not by polymerase chain reaction (PCR) methods. During alpha-thal screening and genotyping of 235 patients diagnosed by laboratory tests hemoglobin (Hb), MCV, MCH and Hb H inclusion bodies] using the gap-PCR method, 175 patients were diagnosed to be carriers of an alpha-thal gene, genotypes of which were 133 alpha-thal-2, 34 alpha-thal-1 (including one only by laboratory test) and eight with Hb H disease. Detection of the alpha(CS) gene for the carriers of alpha-thal-1 and Hb H disease was done by the mismatched PCR-RFLP (restriction fragment length polymorphism) method and the alpha(CS) gene was found in the homozygous state in an alpha-thal-1 patient and a single gene form in two Hb H disease patients. These genotypes were characterized by the PCR-sequencing method. These patients clinically presented the aspects of Hb H disease and of a homozygote form of alpha-thal-1. The description of the alpha(CS) gene in Myanmar is of great value in the development of an effective procedure for prenatal diagnosis of Hb Bart's hydrops fetalis syndrome.