We utilized the PCR method to amplify the alpha-thalassemia-1 breakpoint area of the Southeast Asia type and several regions of the alpha-globin gene cluster to diagnose rightward deletion (-alpha 3.7), leftward deletion (-alpha 4.2) or nondeletion forms of the Hb H disease. For the nondeletion form, a natural restriction site of MseI was used to detect the Hb Constant Spring (Hb CS) or other termination codon mutations. Another naturally occurring restriction site of MspI was used to detect the Hb Quong-Sze. For the deletion form of the Hb H disease, the differences among nonhomologous I, II and III of the rightward or leftward deletion were used to distinguish the mutations. For further characterization of the subtype of -alpha 3.7 deletion, the same primers for detecting termination codon mutations were used to amplify part of the alpha-globin gene, then the PCR product was digested by the restriction enzyme ApaI. In the 57 cases which were studied, 19 were deletion forms while 38 were nondeletion forms. In the deletion form cases, 13 were rightward deletion (-alpha 3.7) and the other 6 were leftward deletion (-alpha 4.2). However, all of the nondeletion form cases were alpha-thalassemia-1 with Hb CS. After the subtyping of -alpha 3.7 deletion, 11 out of 13 were type I deletions and the other 2 were type II deletions.