We report a novel frameshift mutation in exon 3 of the β-globin gene, that, in the heterozygous state, leads to a β-thalassemia intermedia (β-TI) phenotype (marked anemia, splenomegaly, hyperbilirubinemia, jaundice, unbalanced synthesis of α/non-α chains in a 34-year-old Italian woman. This frameshift mutation, due to the deletion of the first nucleotide (-A) at codon 120, results in a β-globin chain that is elongated to 156 amino acid residues. These highly unstable abnormal chains precipitate in the erythroblasts as inclusion bodies, thus causing inefficient erythropoiesis and ultimately resulting in the observed dominant clinical phenotype.