The relative excess of alpha- over beta-globin chains in the erythroid precursors is the chief pathophysiological factor of homozygous beta-thalassemia. The clinical picture is usually characterized by a transfusion-dependent dyserythropoietic anemia (thalassemia major). However, some patients present with moderate anemia that does not require regular blood transfusions (thalassemia intermedia). The molecular heterogeneity of beta-thalassemia mutations and changes of alpha- and gamma-globin gene expression play an important role in modifying the clinical phenotype. We report here on a female Greek patient with homozygous beta-thalassemia but normal growth and development, excellent exercise tolerance, and no need of blood transfusions. She is thus mildly affected clinically, although there is marked pallor, jaundice, and hepatosplenomegaly. These signs correspond to her marked hypochromic, microcytic anemia with erythroid hyperplasia of the bone marrow. beta-Globin genotyping shows here to be compound heterozygous for the codon 39 C-->T beta zero-nonsense mutation and for the T-->C beta(+)-mutation at position 6 of the splice consensus at the exon 1/intron 1 junction (CD39 C-->T/IVS1-6 T-->C). alpha-Globin gene mapping demonstrates the presence of a 3.7-kb alpha (+)-thalassemia deletion on one allele (-alpha 3.7/alpha alpha). Taken together, this study identifies a complex interaction of genetic factors that do not significantly alter the clinical phenotype when present alone but ameliorate the course of homozygous beta-thalassemia when inherited in combination.