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We have identified the β-thalassemia mutations in 59 patients with thalassemia major and 47 patients with Hb E-β-thalassemia, and the deletional and nondeletional α-thalassemia determinants in 23 out of 24 patients with Hb H disease. All persons were attending the Haematology Clinic at the National University of Malaysia in Kuala Lumpur (Malaysia). Most patients (76) were of Malay descent, while 52 patients were Chinese, and two came from elsewhere. The most frequently occurring β -thalassemia alleles among the Malay patients were IVS-I-5 (G→C) and G→A at codon 26 (Hb E), while a few others were present at lower frequencies. The Chinese patients carried the mutation characteristic for Chinese [mainly codons 41/42 (-TTCT) and IVS-II-654 (C→T)]; Malay mutations were not observed among Chinese and Chinese mutations were virtually absent in the Malay patients. The large group of patients with Hb E-β-thalassernia and different β-thaiassemia alleles offered the opportunity of comparing hematological data; information obtained for patients with Hb E-β-thalassemia living in other countries was included in this comparison. Twenty-three patients with Hb H disease carried the Southeast Asian (SEA) α-thalassemia-1 deletion; 13 had the αcsα (Constant Spring) nondeletional α-thalassemia-2 determinant, while the deletional α-thalassemia-2 (-3.7 or -4.2 kb) was present in 10 subjects. The -/αCSα condition appeared to be the most severe with higher Hb H values. Both deletional and nondeletional types of α-thalassemia-2 were seen among Malay and Chinese patients.