Over the past 20 years we have studied 1781 patients with beta-thalassemia syndromes of which 1481 Yugoslav, 166 Bulgarian, 102 Turkish and 32 Albanian. In this paper we summarize the data on the heterogeneity and molecular basis of beta-thal, delta beta-thal and Lepore hemoglobinopathy in these four nationalities living in Yugoslavia and Bulgaria. Beta-thalassemia is the most frequent form of thalassemia in all four nationalities. At the phenotypic level 9 different forms were detected, while at the genotypic level 17 different mutations were characterized. DNA analysis of 377 beta-thal chromosomes showed that in Yugoslav 5 mutations (IVS I#110, IVS I#6, IVS I#1, Codon #39 and Poly A (A----G)) account for 83% of the beta-thal genes, while in Bulgarian 6 mutations (IVS I#110, Codon #39, IVS II#6, IVS II#745, Codon #8 and Codon #8/9) account for 76% of the beta-thal genes. In Turkish the most common mutation is IVS I#110, followed by IVS I#6, IVS II#1 and Codon #8, while in Albanian IVS I#110, Codon #39 and IVS I#6 are prevalent. Delta beta-thalassemia is the second most common form of thal in Yugoslav (10.8%) and Bulgarian (4.2%) but not in Turkish and Albanian. At the phenotypic level 6 different forms were detected, while at the molecular level 3 different deletions (approximately 15 kb, approximately 23 kb, and approximately 148 kb) were characterized. Hb Lepore is the third most common hemoglobinopathy among Yugoslav (9.6%) and most probably Bulgarian (3.67%). This condition was observed in homozygous (n 7) and heterozygous (n 124) state, and in association with beta zero-thal (n 5), beta(+)-thal (n 11), (delta beta)zero-thal (n 1), and Hb Beograd (n 1). Haplotype analysis of 29 lepore chromosomes from 9 unrelated families showed that Hb Lepore is associated with haplotype V.