A total of 38 patients with β-thalassemia intermedia from 30 families were studied. Twelve of the thirty unrelated patients had β°-thalassemia which was due to a homozygosity for one of two different thalassemia defects, namely the frameshift at codon 8, and the IVS-II-1 G→ A mutation. Another mild variation, a β +-thalassemia, was a homozygosity for the mutation of T→C at position 6 of IVS-1 (10 patients). Compound heterozygosities for mild thalassemic determinants or for one mild and one severe β-thalassemic determinant were also found in some patients with β-thalassemia intermedia. The mutations at β-39 and IVS-I-110 were the most commonly occurring thalassemic determinants in these patients. Correlations between genotype and phenotype indicated significant differences in some of the hematological parameters among patients with the IVS-I-6 and the frameshift at codon 8, IVS-I-6 and IVS-II-1, and the frameshift at codon 8 and IVS-II-1 mutations.